Merge pull request #12242 from twbs/sass-contributing

add link to Sass CONTRIBUTING file in the About page

docs/about.html

@@ -145,7 +145,7 @@ lead: "Learn about the history of Bootstrap, meet the core team, and check out t
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-  <p>The <a href="{{ site.sass_repo }}">official Sass port of Bootstrap</a> was created and is maintained by this team. It became part of Bootstrap's organization with v3.1.</p>
+  <p>The <a href="{{ site.sass_repo }}">official Sass port of Bootstrap</a> was created and is maintained by this team. It became part of Bootstrap's organization with v3.1. Read the Sass <a href="https://github.com/twbs/bootstrap-sass/blob/master/CONTRIBUTING.md">contributing guidelines</a> for information on how the Sass port is developed.</p>
 </div>