Add build instructions for the Twitter classifier assembly.

It wasn't clear how to build the assembly documented in the book, so I've added a README file to the project source directory.

I was thinking this might be useful to mention in the book itself, but I am not familiar enough with Gitbooks, nor your conventions to do so with any confidence.

Another option I considered would be to add `assembly.sbt` to `.../twitter_classifier/scala/project` and clean this special case up in the ignore settings.  This would allow building the assembly out of the box (assuming one is using the correct version of SBT).