Add documentation for the post-indexchanged hook

Documentation/githooks.txt

@@ -512,6 +512,24 @@ of the working directory and are separated by a single NUL.  Full paths
 The exit status determines whether git will use the data from the
 hook.  On error, git will abort the command with an error message.
 
+post-indexchanged
+~~~~~~~~~~~~~~~~~
+
+This hook is invoked when the index is written in read-cache.c
+do_write_locked_index.
+
+The first parameter passed to the hook is the indicator for the
+working directory being updated.  "1" meaning working directory
+was updated or "0" when the working directory was not updated.
+
+The second parameter passed to the hook is the indicator for whether
+or not the index was updated and the skip-worktree bit could have
+changed.  "1" meaning skip-worktree bits could have been updated
+and "0" meaning they were not.
+
+Only one parameter should be set to "1" when the hook runs.  The hook
+running passing "1", "1" should not be possible.
+
 GIT
 ---
 Part of the linkgit:git[1] suite