add ignore = dirty to .gitmodules (for helptags)

From http://vimcasts.org/episodes/synchronizing-plugins-with-git-submodules-and-pathogen/::

Generating helptags dirties a submodule’s git repository tree.
Nils Haldenwang has written a blog post describing a simple fix, which
just involves adding the line ignore = dirty to the .gitmodules file for
each submodule that reports a dirty tree when you run git status. Go and
read Nils’s blog post, which goes into a bit more detail.

http://www.nils-haldenwang.de/frameworks-and-tools/git/how-to-ignore-changes-in-git-submodules
update README on how to generate helptags (with :Helptags)