Add Contributor Code of Conduct

[eliheuer]

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 and the Rust CoC.

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.

[eliheuer]

This example of why a CoC might be needed was shared today: #3474

[behdad]

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

This is out-dated. See googlefonts/colr-gradients-spec#300