Update contributing guide
Update contributing guide

Add more information on the team's process for accepting PRs and
formatting of commits and PRs. Add our mailing list. Add more testing
information.

Change-Id: I396d92ee4ff2a1c364bc212d52ba6b644340b8d5

GoogleCloudPlatform/elcarro-oracle-operator,GoogleCloudPlatform/elcarro-oracle-operator