Improve release checklist.

[fenn-cs]

• Release process should start with making a clean clone to ensure consistent tarball sizes across releases.

• The git command(s) needed to obtain new contributors on a particular has issues and did not work for me properly, we want to define a command that works reliable to yield the expected result always.

• We need to add notes about GPG setup and how the signing process works, the current release checklist would be acceptable for someone who already understands the basics about the release process at Tahoe but would be unclear to any other person.