Document prerelease testing floc 1018

Fixes https://clusterhq.atlassian.net/browse/FLOC-1018

That issue states:

> Document this in the release process document (some information is already present in the release versioning document; this should probably be moved).

I haven't done that and don't think that it is right. Since the issue was created, the release process document has been updated to say:

> The version number must adhere to :ref:`the Flocker version numbering policy <version-numbers>`.

I think that this covers the needs appropriately.
I think these last two bits of information need to go somewhere else.  Developers aren't going to be perusing this document as part of their day to day activities - but those are the activities that are going to create this situation.  Let's leave this part out.

The rest (about feature freezes, timing, and release numbering) is good stuff to share with the community.  It lets them know what to expect from the versions we apply to things.