link to html-proofer-docker

README.md

@@ -147,6 +147,10 @@ htmlproofer ./out --url-swap wow:cow,mow:doh --extension .html.erb --url-ignore
 Note: since `url_swap` is a bit special, you'll pass in a pair of `RegEx:String` values.
 `htmlproofer` will figure out what you mean.
 
+#### Using through Docker
+
+If you have trouble with (or don't want to) install Ruby/Nokogiri, the command-line tool can be run through Docker. See [html-proofer-docker](https://github.com/18F/html-proofer-docker) for more information.
+
 ### Using with Jekyll
 
 Want to use HTML Proofer with your Jekyll site? Awesome. Simply add `gem 'html-proofer'`