ignore tags (created by `:Helptags` when using pathogen)

I use pathogen to deal with the vim plugins, each added as a git submodule. To enable properly the help files, I have `Helptags` in my vimrc. 
This create `tags` in the `doc` directory  with each plugin, and if `tags` its not ignored, the submodule results to have `(modified content)`. This should fix it