Add or update instructions for running from source

[ageorgou]

The README has instructions on compiling and building the jar using Maven. The wiki has instructions on launching Nammu from a console. We are missing instructions on how to setup a development environment where you can run the code without building a jar.

The Travis file and the Vagrant setup have two (similar but not identical) sets of instructions, but there are other nuances for a local machine, such as using environments (and getting Jython to recognise them!).