Add link to the "how to contribute" gist to the top-level README of this repo

[nathalie-ckc]

Since this repo doesn't have a CONTRIBUTING.md, we should do this so that potential contributors can immediately know how to get started. Right now, someone in-the-know has to tell you about the gist, which means you're missing out on potential contributors who don't know who to ask.

https://gist.github.com/tkuhrt/10211ae0a26a91a8c030d00344f7d11b

Alternatively, moving the guidelines entirely into a top-level CONTRIBUTING.md in the repo would work too.

[sofiter]

I added the file and made a pull request for this issue.