Improve contribution guidelines

[xtineskim]

From our Cilium community call, there can be some helpful follow up tasks to improve contributions to this repo

• Update the main README.md to have a FAQ or similar on how to find which SIG to get in touch with
• Maybe look into having groups in the community sig groups be taggable in CFP PRs with Github?

[joestringer]

groups in the community sig groups be taggable in CFP PRs with Github

This should already be doable, just need to tag @cilium/<group>. This will trigger GitHub notifications to members of that group.

[xtineskim]

a shortlist from looking and getting inspiration from other CNCF projects:

• Having a shared google drive for docs that are getting feedback (before submitting a PR), allows for better integration with google groups mailing lists.
• Having guidelines on expected review time (ex: design CFPs will be initially reviewed within 2 weeks, PRs will be reviewed within 2 days)

[xmulligan]

Looks like we checked off the first one with #46 and the second one is possible. I think generally in the project we don't provide expectations on really anything for timelines. I think there is this google drive, but I'm not sure how much we use it anymore