(TK-117) Add external API docs

Add some external API docs in the doc folder which detail how
to make requests using the Java and clojure clients.
+1 on the idea of using Javadocs to presenting the documentation for the Java APIs instead of just inlining the documentation here.  Would help for perusing the source code directly as well.  @fpringvaldsen, I'd defer to you as to whether you'd want to do the Javadocs-related work on a separate ticket/PR.