docs/process/release-checklist.txt: Say to push staging branch

docs/process/release-checklist.txt: Say to push staging branch

Preparing a real release, not just an RC, involves making commits.
Typically, those will be on staging-$x.  The tag will refer to them,
and the checklist already says to push them to xenbits.

But if the *branch* is not pushed, then people who just "git fetch"
won't get the tag because it refers to commits they don't have.
(Because of the strange rules git has about tag fetching.)
Worse, the same may be true of people who "git clone".

And anyway, those commits *should* be fed to staging-$x.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>