DOC: update contributing.rst for ci
xref #15878 
cc @bashtage 

should I add the bit about the auto-cancel on a per-branch?  added in 2nd commit. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15880?src=pr&el=h1) Report
> Merging [#15880](https://codecov.io/gh/pandas-dev/pandas/pull/15880?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/456e729384c315c291ef92bb150bcc4f79a22bdf?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/15880/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/15880?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15880      +/-   ##
==========================================
- Coverage   90.98%   90.97%   -0.02%     
==========================================
  Files         143      143              
  Lines       49477    49477              
==========================================
- Hits        45019    45010       -9     
- Misses       4458     4467       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.73% <ø> (ø)` | :arrow_up: |
| #single | `40.63% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15880?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/15880?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/15880?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.56% <0%> (-0.1%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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