[docs] Creates release policy document to discuss

This documents the current release policy as it appears (based on the
Rakefile) but may not be comprehensive.

For github issue #1406
[core] Breaks down rake tasks

While documenting our release process, we can see some things that don't
need to be locked together are.

This splits release into prepare/publish parts

Now anyone can prepare a release but will not be able to publish that
release without the correct access to Github and Rubygems.

Adds a preflight task and adds a check for existing tags to that.