Added information about why and when a prerelease is made
Replaced "will typically" with "should" when referring to when pre-releases are made.
Update release-schedule-versioning.rst

Preceeded > Preceded
Addressed review comment: Pre-releases *should* precede releases, not "must"
Merge branch 'document-prerelease-testing-FLOC-1018' of github.com:ClusterHQ/flocker into document-prerelease-testing-FLOC-1018
Removed info which should go somewhere else
Mandatory.
Merge pull request #1009 from ClusterHQ/document-prerelease-testing-FLOC-1018

Document the one week pre-release testing period.