Adds instructions for including image assets
Adds instructions for including image assets

Some of the MDL components reference image assets that are not imported automatically. This pull request adds instructions to the readme on how to include them using broccoli.
mike-north/ember-material-lite,truenorth/ember-material-lite,truenorth/ember-material-lite,truenorth/ember-material-lite,mike-north/ember-material-lite,mike-north/ember-material-lite