add .gitignore to exclude doc/tags

[takedawilliam]

When using the plugin, Vim automatically created doc/tags to store help information.

In the below scenario, dotfiles is where the main git repo locates. In case users use

git submodule add plugin_repo_link vim/bundle/loremipsum

they cannot perform git commit in dotfiles. The following message is displayed when doing git status

modified: vim/bundle/loremipsum (untracked content)

The directory tree
dotfiles
├── README.md
├── _vimrc
└── vim
   ├── autoload
   ├── bundle
   │   ├── loremipsum
   │   │   ├── README
   │   │   ├── autoload
   │   │   ├── doc
   │   │   └── plugin
   ├── colors
   ├── plugin
   └── sessions