Ignore '/doc/tags'

Most vim plugins ignore `/doc/tags` since it's a generated file

###### References

- https://github.com/gmarik/Vundle.vim/blob/master/.gitignore
- https://github.com/sjl/gundo.vim/blob/master/.gitignore
- https://github.com/scrooloose/nerdtree/blob/master/.gitignore
- https://github.com/godlygeek/tabular/blob/master/.gitignore
- *et cetera*