Add documentation on how to setup the project

[smarr]

Document how to:

• use BD in an existing Truffle language
• how to develop BD, use CI, etc

[ghost]

To add to the list here, I think it would be great to have a bare-bones "hello world" style of example. Obviously, the example needs to be a full interpreter, so it's not going to that simple. However, I think it's important to have a simple starting point for new users. One option would be for us to fork Simple Language and replace the code with diamonds where we can.

I'm not sure if we will ever have enough free time at the same time, but perhaps we (@smarr, @charig, @daumayr, @Richard-Roberts) could do this together somehow - two people take notes while the other two people do the implementation. It shouldn't take too long (one weekend perhaps) and through doing this we generate good documentation?