add minimal addon-info.json file

This is needed at Google to prevent vim-go from conflicting with our internal vim plugin for go syntax (basically the misc/vim files).  I'm not super familiar with addon-info.json files myself, but a number of vim plugin managers seem to be supporting this format, so it seems like a good thing to add.

I debated whether to name it "vim-go" or "go", and landed on the latter since it is really intended to be a complete replacement for the vim files that ship with Go.  I don't have strong feelings about that though, and can switch it back to "vim-go" if you'd like.