Documentation/git-merge: document subtree strategy.

There was already some documentation about subtree under
Documentation/howto but it was missing from git-merge manpage.
Signed-off-by: NMiklos Vajna <vmiklos@frugalware.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/merge-strategies.txt

@@ -33,3 +33,10 @@ ours::
 	merge is always the current branch head.  It is meant to
 	be used to supersede old development history of side
 	branches.
+
+subtree::
+	This is a modified recursive strategy. When merging trees A and
+	B, if B corresponds to a subtree of A, B is first adjusted to
+	match the tree structure of A, instead of reading the trees at
+	the same level. This adjustment is also done to the common
+	ancestor tree.