Add developer docs for creating releases

Provide an initial outline on the steps Release Managers follow for creating, voting and publishing a release for Apache Cassandra.

ASF has the following guidelines:

• `ASF Release Policy <http://www.apache.org/legal/release-policy.html>`_.
• `ASF Release Distribution Policy <http://www.apache.org/dev/release-distribution>`_.
• `ASF Release Best Practices <http://www.eu.apache.org/dev/release-publishing.html>`_.

The project is still doing some things in an outdated manner, eg using people.apache.org URLs for staging artefacts. There is no urgent need to fix these things but by having the docs published it can improved incrementally over time.

fyi mshuler