Create issue and PR management guildlines for the sig contribex
Create issue and PR management guildlines for the sig contribex

These guidelines and responsibilities are focused on  issues and PRs
in the community repo. Keeping it as short as possible for easier usage. It's
also utilizing and poiting to other existing materials on triage.
Add the triage team members and specific role descriptions in a separate
document.

Related #3364

pwittrock/community,kubernetes/community,pwittrock/community,kubernetes/community,kubernetes/community