Update README with heroku deployment issue (cedar and node-sass bug)

Author: mbreining

README.md

@@ -174,4 +174,11 @@ buildpack:
 heroku config:add BUILDPACK_URL=https://github.com/ddollar/heroku-buildpack-multi.git
 ```
 
-This runs the two buildpacks in the `.buildpacks` directory.
+This runs the two buildpacks in the `.buildpacks` file.
+
+Also make sure you are running the latest heroku stack, cedar-14, to avoid running
+into the [following issue](https://github.com/sass/node-sass/issues/467#issuecomment-61729195).
+
+```
+heroku stack:set cedar-14 -a react-webpack-rails-tutorial
+```