Fixes bug 1010032 - Added documentation about Elasticsearch and features...

... based on it.

r? anyone who feels like reading it, the more eyes I get on this the better. I'm mainly thinking about @phrawzty, @peterbe and @bramwelt. 
Instead of having an empty header here, each of the sub-headers could be changed to things like:
* Back-end Resources
* Back-end Processor
* Back-end Middleware