DOC: Additions/updates to documentation
- Minor grammar fixes
- Added some extra details
- Added link to LICENSE
- Made explicit HTTPS calls
- Minor URL cleanup

@alanyee : Thanks for this!  Should be ready to merge on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17150?src=pr&el=h1) Report
> Merging [#17150](https://codecov.io/gh/pandas-dev/pandas/pull/17150?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/611d29606263d12ecdcc38a2b8a790e99aa443d6?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17150/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17150?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17150      +/-   ##
==========================================
- Coverage   91.05%      91%   -0.05%     
==========================================
  Files         161      161              
  Lines       49417    49417              
==========================================
- Hits        44995    44974      -21     
- Misses       4422     4443      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.78% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.28% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17150?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17150?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17150?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17150?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.66% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17150?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/17150?src=pr&el=footer). Last update [611d296...d545d44](https://codecov.io/gh/pandas-dev/pandas/pull/17150?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!