DOC: ecosystem.rst: Koalas
- [x] closes #26424
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I (almost) copied and pasted from its README.md.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=h1) Report
> Merging [#26425](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cb6e47d5a04c89cb6f3564753c43abadbdf81c94?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26425/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26425      +/-   ##
==========================================
- Coverage   91.69%   91.69%   -0.01%     
==========================================
  Files         174      174              
  Lines       50741    50741              
==========================================
- Hits        46529    46525       -4     
- Misses       4212     4216       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.19% <ø> (ø)` | :arrow_up: |
| #single | `41.16% <ø> (-0.18%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26425/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26425/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=footer). Last update [cb6e47d...d0063ba](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=h1) Report
> Merging [#26425](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cb6e47d5a04c89cb6f3564753c43abadbdf81c94?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26425/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26425      +/-   ##
==========================================
- Coverage   91.69%   91.69%   -0.01%     
==========================================
  Files         174      174              
  Lines       50741    50741              
==========================================
- Hits        46529    46525       -4     
- Misses       4212     4216       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.19% <ø> (ø)` | :arrow_up: |
| #single | `41.16% <ø> (-0.18%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26425/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26425/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=footer). Last update [cb6e47d...d0063ba](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=h1) Report
> Merging [#26425](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1263e1a9cac179f453aa6223f00cbaa3e158a878?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26425/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26425      +/-   ##
==========================================
- Coverage   91.73%   91.72%   -0.01%     
==========================================
  Files         174      174              
  Lines       50741    50741              
==========================================
- Hits        46548    46544       -4     
- Misses       4193     4197       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <ø> (ø)` | :arrow_up: |
| #single | `41.7% <ø> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26425/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26425/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=footer). Last update [1263e1a...1b0abec](https://codecov.io/gh/pandas-dev/pandas/pull/26425?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Sure, will do. Thanks! Thank you, @datapythonista, @WillAyd and @TomAugspurger for your guidance and approval.