Ignore the doc/tags file created by Pathogen's :Helptags

Hey,

Here's a trivial commit to make using `vim-projecroot` easier with git submodules/Pathogen. Basically this file is created when running `:Helptags` which dirties the repo (which is a bit annoying when the repo is a submodule). Hopefully that makes sense :).

Thanks,
Alexis