rustc not automatically registered with syntastic

[klnusbaum]

I recently installed rust.vim via vim-plug. When I did, I was disappointed to find that I was not getting any syntax checking. It wasn't until I added the following to my init.vim that I was able to get syntax checking on buffer write:

let g:syntastic_rust_checkers = ['rustc']

[klnusbaum]

I have a feeling you might actually always need to do this (i.e. it's not advisable to add something like the above line automatically without the user knowing). If that's the case, we should update the README to indicate this. Happy to make a small PR if that's the best course of action.

[da-x]

A pull request is most welcomed! Thanks