docs: describe pre-rebase hook

Documentation/git-rebase.txt talks about pre-rebase hook, but it
appears that Documentation/git-hooks.txt does not have corresponding
entry for it.

Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Documentation/githooks.txt

@@ -130,6 +130,13 @@ parameter, and is invoked after a commit is made.
 This hook is meant primarily for notification, and cannot affect
 the outcome of 'git-commit'.
 
+pre-rebase
+----------
+
+This hook is called by 'git-rebase' and can be used to prevent a branch
+from getting rebased.
+
+
 post-checkout
 -----------