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).

docs/_config.yml

@@ -7,6 +7,7 @@ exclude:
   - bin
   - CNAME
   - Gemfile*
+  - vale-styles
   - vendor
 
 plugins:

docs/vale-styles/Homebrew/Abbreviations.yml

@@ -0,0 +1,12 @@
+---
+extends: substitution
+message: Use '%s'
+ignorecase: false
+link: 'https://github.com/Homebrew/brew/blob/master/docs/Prose-Style-Guidelines.md#style-and-usage'
+level: error
+nonword: true
+swap:
+  '\beg\b': e.g.
+  '\bie\b': i.e.
+  'e\.g\.,': e.g.
+  'i\.e\.,': i.e.

docs/vale-styles/Homebrew/OxfordComma.yml

@@ -0,0 +1,8 @@
+---
+extends: existence
+message: 'No Oxford commas!'
+link: 'https://github.com/Homebrew/brew/blob/master/docs/Prose-Style-Guidelines.md#typographical-conventions'
+scope: sentence
+level: warning
+tokens:
+  - '(?:[^,]+,){1,}\s\w+,\sand'

docs/vale-styles/Homebrew/Pronouns.yml

@@ -0,0 +1,15 @@
+---
+extends: existence
+message: Avoid gender-specific language when not necessary.
+link: 'https://github.com/Homebrew/brew/blob/master/docs/Prose-Style-Guidelines.md#personal-pronouns'
+level: warning
+ignorecase: true
+tokens:
+  - him
+  - her
+  - she
+  - he
+  - his
+  - hers
+  - himself
+  - herself

docs/vale-styles/Homebrew/README.md

@@ -0,0 +1 @@
+Based on Homebrew's [Prose Style Guidelines](http://docs.brew.sh/Prose-Style-Guidelines.html).

docs/vale-styles/Homebrew/Spacing.yml

@@ -0,0 +1,9 @@
+---
+extends: existence
+message: "'%s' should have one space."
+link: 'https://github.com/Homebrew/brew/blob/master/docs/Prose-Style-Guidelines.md#typographical-conventions'
+level: error
+nonword: true
+tokens:
+  - '[a-z][.?!][A-Z]'
+  - '[.?!] {2,}[A-Z]'

docs/vale-styles/Homebrew/Terms.yml

@@ -0,0 +1,9 @@
+---
+extends: substitution
+message: Use '%s' instead of '%s'.
+link: 'https://github.com/Homebrew/brew/blob/master/docs/Prose-Style-Guidelines.md#terminology-words-and-word-styling'
+level: error
+swap:
+  Pull Request: pull request
+  repo: repository
+  Rubocop: RuboCop

docs/vale-styles/Homebrew/Titles.yml

@@ -0,0 +1,6 @@
+---
+extends: capitalization
+message: "'%s' should be in sentence case"
+level: warning
+scope: heading
+match: $sentence

docs/vale-styles/Homebrew/Trademarks.yml

@@ -0,0 +1,12 @@
+---
+extends: existence
+message: 'No "TM", ™, SM, ©, ®, or other explicit indicators of rights ownership or trademarks'
+link: 'https://github.com/Homebrew/brew/blob/master/docs/Prose-Style-Guidelines.md#typographical-conventions'
+level: error
+nonword: true
+tokens:
+  - \bTM\b
+  - ™
+  - \bSM\b
+  - ©
+  - ®