Add umount call to init to unmount a filesystem

Bug: 29278988
Change-Id: Ia14ab1bcad9513688df97f7b4d9162a9ab282480
diff --git a/init/readme.txt b/init/readme.txt
index ef85ccf..4481e24 100644
--- a/init/readme.txt
+++ b/init/readme.txt
@@ -355,6 +355,9 @@
    Trigger an event.  Used to queue an action from another
    action.
 
+umount <path>
+   Unmount the filesystem mounted at that path.
+
 verity_load_state
    Internal implementation detail used to load dm-verity state.