Socket REPL guide

[danielcompton]

It could be good to expand docs on the socket REPL to:

• Explain motivations for creating the socket REPL
• Explain how it can be used or extended
• Explain how it is different from the current Clojure REPL, and nREPL
• Discuss the streaming nature of the REPL

There is some good information in the design doc that could be collated for this.