Add license

[fwalch]

I would have sent a PR, but I was not sure whether to use "pure" Apache or the Apache/Vim combination used by neovim/neovim, since there were changes to neovim.rb by people who didn't sign the CLA (albeit neovim.rb was never part of Vim, so I'm not sure what applies here).

[justinmk]

How are homebrew formulae usually licensed, if at all?

[fwalch]

Looking at a few repositories under https://github.com/Homebrew, most of them don't have a LICENSE file; homebrew-nginx uses MIT License.