docs: add webrick gem, generation info

docs/Gemfile

@@ -6,4 +6,5 @@ group :test do
   gem "html-proofer"
   gem "mdl"
   gem "rake"
+  gem "webrick"
 end

docs/README.md

@@ -0,0 +1,17 @@
+# Homebrew Docs
+
+These are the source files for the [Homebrew documentation site](https://docs.brew.sh/).
+
+A [GitHub Action](https://github.com/Homebrew/brew/blob/master/.github/workflows/docs.yml) is run to validate each change before the site is deployed to GitHub Pages.
+
+## Usage
+
+Open <https://docs.brew.sh> in your web browser.
+
+To instead generate the site locally to <http://localhost:4000>, run:
+
+```bash
+cd `brew --repository`/docs
+bundle install
+bundle exec jekyll serve --watch
+```