Document -i flag to git-read-tree

Somehow I missed it when we updated read-tree to support the recursive
merge strategy.  Also -i should require -m as well, which the command
did not check.

Signed-off-by: Junio C Hamano <junkio@cox.net>

read-tree.c

@@ -655,7 +655,7 @@ int main(int argc, char **argv)
 			die("failed to unpack tree object %s", arg);
 		stage++;
 	}
-	if (update && !merge)
+	if ((update||index_only) && !merge)
 		usage(read_tree_usage);
 	if (merge && !fn) {
 		if (stage < 2)