Document the release process.

[nathan-at-least]

Add a description in ./README.rst about the release process, which is roughly:

• Ensure all tickets for the target release milestone are closed except for the release capstone ticket which is always titled: Release N.N
• Ensure the unittest coverage looks good.
• Edit ./setup.py to set the version to N.N, and commit this.
• Install this on a staging test system and do some (FIXME: Which?) smoke tests.
• Tag the revision as release-N.N and push the tag to github.
• Run: ./setup.py sdist --upload --sign --identity $YOUR_CODE_SIGNING_PGP_ID
• Create a new virtual env and run pip install leastbot. Check the installation was successful and has the right version (FIXME: How?).
• Close the capstone ticket, which is the only remaining ticket in the milestone.
• Bonus: Send announcements.

[nathan-at-least]

This is now on master; closing.