Propose new RFC for migrating off Sympa.

Heads up @nathenharvey 

I will post to the mailing list (hehe) and hopefully we can get some comments & have this reviewed by the next community meeting.
Sure, we could create a new `chef-announce` Google Group, outside of the scope of this RFC. I wouldn't bother to migrate what's in `opscode-announce` now. Agree/disagree?