Add note about Jekyll version per #8195

README.md

@@ -30,7 +30,7 @@ Bootstrap's documentation, included in this repo in the `/docs` directory, is bu
 
 ### Running documentation locally
 
-1. If necessary, [install Jekyll](http://jekyllrb.com/docs/installation).
+1. If necessary, [install Jekyll](http://jekyllrb.com/docs/installation) (requires v1.x).
 2. From the `/bootstrap` directory, run `jekyll serve` in the command line.
 3. Open [http://localhost:9001](http://localhost:9001) in your browser, and voilà.