Formalize contributing guide about ebpf

[ti-mo]

@joestringer Thanks for dropping an issue!

I've found some more example contribution guidelines here: https://docs.github.com/en/free-pro-team@latest/github/building-a-strong-community/setting-guidelines-for-repository-contributors#examples-of-contribution-guidelines. The guidelines here don't need to be too elaborate, but could specify things that would otherwise be mentioned during review.

The community member on the call asked about:

• whether accompanying tests need to be contributed in order for a patch to be accepted
• commit/code style