Updated README to follow the changes of the recent Symfony versions

These are the main proposed changes:

  * Updated installation instructions to follow the best practices proposed for reusable bundles (see http://symfony.com/doc/current/cookbook/bundles/best_practices.html#installation-instructions)
  * Use `AppBundle` instead of `Acme\HelloBundle` (this change is right now being made in all the rest of Symfony documentation)
  * Replaced `Symfony2` by `Symfony` (again, this is being made in the rest of the documentation)
  * Use the new password encoder introduced in Symfony 2.6
  * Removed the useless `@inheridoc` PHPdoc blocks