Tell people to //= require angular-rails-templates

In my experiences with angular-rails-templates, I've had to add this to my file that I do something like

```javascript
App = angular.module("app", ["templates"]);
```

It clears up the issue, like in #28 of 
"Module 'templates' is not available" 

My findings is that the instruction should be in the readme.

It looks like it was, but got removed here: https://github.com/pitr/angular-rails-templates/commit/fa7230a10a36835cab911766d03986c89ff640ce#diff-04c6e90faac2675aa89e2176d2eec7d8L18