Add doc/tags to .gitignore (based on Vim 8 documentation)

[nertzy]

I noticed in #32 that you rejected adding doc/tags to .gitignore, because you believe that people should use non-git-checkout directories as their plugins to Vim.

As of Vim 8, the packages-create documentation suggests that people should be able to git clone vim plugin repositories directly into their package directories.

Quote:

This allows for the user to do: 
        mkdir ~/.vim/pack/myfoobar
        cd ~/.vim/pack/myfoobar
        git clone https://github.com/you/foobar.git

I believe that this justifies adding doc/tags to .gitignore in this project. What do you think?

Thanks for maintaining this plugin!

[kana]

Okay, I merged this: 41a675d