Add block settings + update rss example

This is related to:
* https://github.com/symfony-cmf/BlockBundle/pull/10
* https://github.com/symfony-cmf/cmf-sandbox/pull/147, a reference is already added to the FeedBundle but can be removed if this PR is merged earlier
* https://github.com/sonata-project/SonataBlockBundle/pull/43, in bundles/block.rst:
  * added an example in the configuration section
  * mentioned in the configuration cascading explanation of the subsection _default settings_ of the section _Block Service_