created .gitignore, added /doc/tags

For those that use git to manage their vim dotfiles with many submodules, but also have plugins that will automatically create ctags for all plugin docs, not having a `.gitignore` file to manage the automatically generated ctags file in `/doc/tags` creates an annoying mismatch between the local repository and your origin remote. We can either include the /doc/tags file or ignore it in `.gitignore`; this seemed like the less intrusive.