Help doc on GOLEM REPL initialisation

[afsalthaj]

@noise64

@jdegoes has this idea of having a help doc on golem REPL initialization.

Welcome to the Golem REPL!
To get started, you need to create an instance of a component. The REPL currently has access to these components:
 - acme:xyz
To create an instance, use the instance function, which accepts an optional component package (in square brackets) and the name of the instance to create or connect with. For example:
let worker = instance("my-worker");
From here, you can invoke functions on your instance with dot syntax:
worker.inc();