Rss block example

Added an rss block example

Related to this are:
* https://github.com/symfony-cmf/symfony-cmf-website/pull/13 (merged)
* https://github.com/symfony-cmf/BlockBundle/pull/26

And the example in the docs at http://symfony.com/doc/master/cmf/bundles/block.html#create-your-own-blocks can be updated after this is merged.