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: NJunio C Hamano <junkio@cox.net>

Documentation/git-read-tree.txt

@@ -9,7 +9,7 @@ git-read-tree - Reads tree information into the directory cache
 
 SYNOPSIS
 --------
-'git-read-tree' (<tree-ish> | [-m [-u]] <tree-ish1> [<tree-ish2> [<tree-ish3>]])
+'git-read-tree' (<tree-ish> | [-m [-u|-i]] <tree-ish1> [<tree-ish2> [<tree-ish3>]])
 
 
 DESCRIPTION
@@ -35,6 +35,16 @@ OPTIONS
 	After a successful merge, update the files in the work
 	tree with the result of the merge.
 
+-i::
+	Usually a merge requires the index file as well as the
+	files in the working tree are up to date with the
+	current head commit, in order not to lose local
+	changes.  This flag disables the check with the working
+	tree and is meant to be used when creating a merge of
+	trees that are not directly related to the current
+	working tree status into a temporary index file.
+
+
 <tree-ish#>::
 	The id of the tree object(s) to be read/merged.
 

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)