DOC: update contributing.rst for ci

[jreback]

xref #15878

[jreback]

cc @bashtage

should I add the bit about the auto-cancel on a per-branch?

[jreback]

added in 2nd commit.

[codecov]

Codecov Report

Merging #15880 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            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)	⬆️
#single	40.63% <ø> (-0.11%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.56% <0%> (-0.1%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 456e729...498973e. Read the comment docs.