Add Contributor Code of Conduct

Last week in a meeting I heard some complaints about community behavior, so maybe adding a code of conduct to https://github.com/google/fonts might be a good idea? I'm proposing:

- Add a CODE_OF_CONDUCT.md file to the root directory
- Add a link and a brief mention of the above file to the main README.md

Some examples are the [Contributor Covenant](https://www.contributor-covenant.org/) and the [Rust CoC](https://www.rust-lang.org/policies/code-of-conduct).

Some other Google open source project that have done this already:

https://github.com/google/closure-compiler
https://github.com/google/go-cloud

And from the the Google open source docs:

https://opensource.google/docs/releasing/template/CODE_OF_CONDUCT/

I can make a PR adding this, I'll just wait a bit for any feedback on this issue first.