Add a note about Middleman Render Time

Adds a note to the README section for time it takes for Middleman
to render the www-gitlab-com.

README.md

@@ -168,6 +168,8 @@ internet connection available.
 4. Connect to http://localhost:4567
+> **Note**: You won't be able to connect immediately. Middleman takes a few minutes to render the site.
+
 5. Change your original content as usual, and see the changes in the browser as soon as
 you save the new version of the file (otherwise, just restart the container)