Document EXECJS_RUNTIME flag workaround.

README.md

@@ -20,6 +20,19 @@ PDF files are not available in development mode. See below for more information.
 See the [Middleman docs](https://middlemanapp.com/basics/development_cycle/) for
 more information.
+### A known issue with Middleman in local development
+
+There's a [known issue](https://gitlab.com/gitlab-com/www-gitlab-com/issues/842)
+that causes the Middleman server to hang indefinitely when changes are made to 
+certain filetypes. For the time being, you can workaround the issue in development 
+by explicitly setting `EXECJS_RUNTIME` before starting the Middleman server:
+
+```sh
+bundle install
+
+EXECJS_RUNTIME=Node bundle exec middleman
+```
+
 ### Enable livereloading
 When running middleman with the livereload option enabled, it watches your