doc update: mention conflict with handlebars_assets

having the handlebars_assets gem in along with ember-rails will cause an unending fury of frustration since none of the templates will ever get picked up and you wont know why

a few ideas to make this little conflict less painful to people adding ember-rails to an existing project:

**low effort:** add a note in the readme recommending to remove `handlebars_assets` when using ember-rails

**medium effort:** add runtime check with error message

**high effort:** allow both gems to work happily together

I can possibly tackle the low/medium effort items. high effort one is probably not required as long as we're clear that handlebars_assets and ember-rails are Gemfile mortal enemies