Update readme

Update the README to reflect the codegen situation.
As noone is able to tell me what the status of the Java code generator is, I cannot make any statement on it. It is not optional, and built by default. 

#1127 is out of date, and CodegenJava should just be treated as CodegenLLVM is.

This should be done by a person who cares about it (which, unfortunately, isn't me). Everyone wanting to rewrite the README in another way is free to do so, and/or submit a PR. It's a waste of time discussing this at length.