[FLINK-1107] Hadoop Compatibility Layer documented

This PR contains the Hadoop Compatibility Layer documentation and some small typo fixes. 

Maybe it would make sense to introduce a special "Add-ons" top-level section instead of putting it under "Programming Guides"?
can you add another headline here, that quickly explains the usage?
Just how to read data from the environment to a data set using a hadoop input format (something like a copy-past skeleton).
I think its better if you have a) a general definition how to use it
and b) an example that people see how to use it in context.