Add gitignore so that vim-plug can be managed by vim-plug

[smuenzel]

If you set up vim-plug to manage vim-plug itself, it will create a doc/tags file, which means the repo it is saved in is dirty. Fix this by adding a gitignore.

[junegunn]

I have to mention this:

• https://github.com/junegunn/vim-plug/wiki/faq#shouldnt-vim-plug-update-itself-on-plugupdate-like-vundle
• Allow vim-plug to be managed as a plugin #240 (comment)

But, having a .gitignore file wouldn't hurt.