add .gitignore to excluse doc/tags

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** 
> <span style="color: red">modified:</span> vim/bundle/loremipsum (untracked content)

**The directory tree**
dotfiles
├── README.md
├── _vimrc
└── vim
    ├── autoload
    │   └── pathogen.vim
    ├── bundle
    │   ├── loremipsum
    │   │   ├── README
    │   │   ├── autoload
    │   │   ├── doc
    │   │   └── plugin
    ├── colors
    ├── eclim
    ├── plugin
    └── sessions