DOC: Add CoC to the README
@datapythonista : Add CoC to README file
Sorry about the CI failures again @sparalic. If you merge master and push, things should be fixed

```
$ git fetch upstream
$ git merge upstream/matser
$ # git add and commit stuff
$ git push origin pandas-coc
```

You can also put in @gfyoung's request change at the same time if you want @TomAugspurger  and @gfyoung thank you for all of your help. @TomAugspurger, I tried merging with the master and pushing as you suggested, but some of the CI test failed again. @sparalic : I restarted the failing build for you to see if the failure goes away.  If not, you should merge with `master` and push. @datapythonista and @gfyoung thank you! You are just not having luck with our CI @sparalic 😄  The windows failure is unrelated. Tracking it in https://github.com/pandas-dev/pandas/issues/27902.

This is good to merge once the doc build passes. Thanks @sparalic!