Add Vale linting for our docs

- I noticed that another open source project had taken the Homebrew
  style guide and made it into [Vale linting
  rules](https://github.com/testthedocs/vale-styles/tree/master/Homebrew).
  This copies them into here, so that we can make use of them. Thanks!
- What use is a style guide if our own docs don't at least try to
  adhere to it? This aims to make the rules more visible to all
  contributors.
- In order for these to do anything, you'll have to `brew install vale`
  and run `vale` in the root of this repo. It will look for Markdown
  files.
- A next step would be adding this as a pre-commit hook, or...
- The GitHub Action would have been really good, but [it doesn't support
  forks](https://github.com/errata-ai/vale-action#limitations).