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.
Another "should" here.