Submodule gotcha when manually adding.

Don't append the trailing slash, otherwise, bad things will happen.

Signed-off-by: Scott Chacon <schacon@gmail.com>

text/34_Git_Submodules/1_Submodules.markdown

@@ -181,6 +181,48 @@ others won't be able to clone the repository:
     Did you forget to 'git add'?
     Unable to checkout '261dfac35cb99d380eb966e102c1197139f7fa24' in submodule path 'a'
 
+If you are staging an updated submodule for commit manually, be careful to not
+add a trailing slash when specifying the path. With the slash appended, Git
+will assume you are removing the submodule and checking that directory's
+contents into the containing repository.
+
+    $ cd ~/git/super/a
+    $ echo i added another line to this file >> a.txt
+    $ git commit -a -m "doing it wrong this time"
+    $ cd ..
+    $ git add a/
+    $ git status
+    # On branch master
+    # Changes to be committed:
+    #   (use "git reset HEAD <file>..." to unstage)
+    #
+    #       deleted:    a
+    #       new file:   a/a.txt
+    #
+    # Modified submodules:
+    #
+    # * a aa5c351...0000000 (1):
+    #   < Initial commit, submodule a
+    #
+
+To fix the index after performing this operation, reset the changes and then
+add the submodule without the trailing slash.
+
+    $ git reset HEAD A
+    $ git add a
+    $ git status
+    # On branch master
+    # Changes to be committed:
+    #   (use "git reset HEAD <file>..." to unstage)
+    #
+    #       modified:   a
+    #
+    # Modified submodules:
+    #
+    # * a aa5c351...8d3ba36 (1):
+    #   > doing it wrong this time
+    #
+
 You also should not rewind branches in a submodule beyond commits that were
 ever recorded in any superproject.