mention bower in getting started section

So that I don't miss this again ;)

https://github.com/endlessinc/ember-restless/issues/51

Note: Many lines in diff because trailing whitespace was removed, only these lines are relevant:

https://github.com/bguiz/ember-restless/compare/update-getting-started?expand=1#diff-04c6e90faac2675aa89e2176d2eec7d8R39