Document that Protractor requires JDK

[jakenbuhler]

The error message resulting from a missing JDK is somewhat cryptic:

------------------------------------
PID: 20518 (capability: chrome #1)
------------------------------------

Starting selenium standalone server...

events.js:72
         throw er; // Unhandled 'error' event
          ^
Error: spawn ENOENT
    at errnoException (child_process.js:998:11)
    at Process.ChildProcess._handle.onexit (child_process.js:789:34)
[launcher] Runner Process Exited With Error Code: 8

It appears that I'm not the only person that's run into this (#200). It would be helpful to mention the JDK requirement in the docs.

[petebacondarwin]

@jrothenbuhler - do you fancy putting together a PR on the README for this?

[jakenbuhler]

Yeah no problem, I'll take a stab at it.