ENH: Citing source in README file
For GH users who strictly or heavily use the web-view instead of a local Git, having a direct link is handy, as it does not require downloading the PDF _if_ the user wanted to go to the source of it directly. It's an alternative that allows those interested in more uploads similar to this PDF from the same author(s).

 - [ ] closes: N/A
 - [ ] tests added / passed: N/A
 - [ ] whatsnew: Added source of PDF to README file for the cheatsheet

You linked to the R cheatsheet (on which the pandas one is based). Was that on purpose? actually I think this is reasonable, though I think the wording should be that this is *referenced/based* on the dplyr sheet. @jorisvandenbossche Good catch! I've updated the file with a new commit.
@jreback feel free to word this to whatever is most appropriate. As long as the spirit of the PR is satisfied, I'm happy with whatever implementation is used. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=h1) Report
> Merging [#15856](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1b53d8864af0ed936f84d0935e2cc360dc9f8de7?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15856/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15856      +/-   ##
==========================================
- Coverage   90.98%   90.97%   -0.02%     
==========================================
  Files         143      145       +2     
  Lines       49443    49483      +40     
==========================================
+ Hits        44985    45015      +30     
- Misses       4458     4468      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.73% <ø> (ø)` | :arrow_up: |
| #single | `40.63% <ø> (-0.13%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/util/clipboard/windows.py](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvY2xpcGJvYXJkL3dpbmRvd3MucHk=) | `11.62% <0%> (-1.02%)` | :arrow_down: |
| [pandas/io/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=tree#diff-cGFuZGFzL2lvL2NvbW1vbi5weQ==) | `69.33% <0%> (-0.93%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.03% <0%> (-0.27%)` | :arrow_down: |
| [pandas/computation/engines.py](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=tree#diff-cGFuZGFzL2NvbXB1dGF0aW9uL2VuZ2luZXMucHk=) | `88.88% <0%> (-0.18%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.56% <0%> (-0.1%)` | :arrow_down: |
| [pandas/util/clipboard/clipboards.py](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvY2xpcGJvYXJkL2NsaXBib2FyZHMucHk=) | `26.58% <0%> (ø)` | :arrow_up: |
| [pandas/io/feather\_format.py](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=tree#diff-cGFuZGFzL2lvL2ZlYXRoZXJfZm9ybWF0LnB5) | `86.66% <0%> (ø)` | :arrow_up: |
| [pandas/\_version.py](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=tree#diff-cGFuZGFzL192ZXJzaW9uLnB5) | `44.65% <0%> (ø)` | :arrow_up: |
| [pandas/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=tree#diff-cGFuZGFzL19faW5pdF9fLnB5) | `92.68% <0%> (ø)` | :arrow_up: |
| ... and [20 more](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15856?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/15856?src=pr&el=footer). Last update [1b53d88...735fd94](https://codecov.io/gh/pandas-dev/pandas/pull/15856?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). seems reasonable, thanks.