Update README.md

added instruction to `git checkout master` to activate the hooks and init/update the submodules.

git-hooks/README.md

@@ -7,3 +7,4 b' both of which just update submodules,'
 so make sure that you have a fully synced repo whenever you checkout or pull.
 
 To use these hooks, run `./install-hooks.sh`.
+If you havn't initialised and updated the submodules manually, you will then need to run `git checkout master` to activate the hooks (even if you already have `master` checked out).