Document Reducing Heroku Slug size

Closes #491.

[Heroku slug size is limited](https://devcenter.heroku.com/articles/slug-compiler#slug-size). The build process creates artifacts that are not necessary for the server to run, but are included in the deployed Heroku slug.

Omitting these build assets can dramatically reduce slug size.

A build-pack solution for this is discussed in [Issue #491][#491].

[#491]: #491

Author: Nicholas Rutherford

README.md

@@ -278,6 +278,16 @@ applications into the project.
 
 [buildpack]: https://devcenter.heroku.com/articles/using-multiple-buildpacks-for-an-app#adding-a-buildpack
 
+#### Slug size
+
+[Heroku slug size is limited](https://devcenter.heroku.com/articles/slug-compiler#slug-size). The build process creates artifacts that are not necessary for the server to run, but are included in the deployed Heroku slug.
+
+Omitting these build assets can dramatically reduce slug size.
+
+A build-pack solution for this is discussed in [Issue #491][#491].
+
+[#491]: https://github.com/thoughtbot/ember-cli-rails/issues/491
+
 ### Capistrano
 
 EmberCLI-Rails executes both `npm install` and `bower install` during EmberCLI's
@@ -304,7 +314,7 @@ contains the directory or directories that contain the `bower` and `npm`
 executables.
 
 #### For faster deployments
-Place the following in your deploy/<environment>.rb 
+Place the following in your deploy/<environment>.rb
 ```ruby
 set :linked_dirs, %w{<ember-app-name>/node_modules <ember-app-name>/bower_components}
 ```