Add hints to parallel-letter-frequency exercise

exercises/parallel-letter-frequency/HINTS.md

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+## Hints
+According to [this terminology](http://chimera.labs.oreilly.com/books/1230000000929/ch01.html#sec_terminology) you should write a *parallel* and *deterministic*
+program and (by all means!) let Scala deal with the *concurrency* aspect.
+Or else your code could quickly get messy and error-prone with all kinds of nasty
+concurrency bugs. In particular your program could become indeterministic
+which spells in practice: very (in fact, VERY) hard to debug, test and reason about.
+
+Having said that it might be a good idea to first write a sequential solution (and
+use the test suite to verify it).
+Only then should you try to parallelize it while keeping the sequential and parallel
+portions of your code as separate as possible.
+
+A first iteration could be using Scala's [parallel collections](http://docs.scala-lang.org/overviews/parallel-collections/overview.html). You might find
+that this is almost too simple (especially if you have followed our advice and
+already have a sequential solution).
+
+For the second iteration we recommend you try a solution with [scala.concurrent.Future](http://www.scala-lang.org/api/current/scala/concurrent/Future$.html).
+You can consult [this tutorial](http://danielwestheide.com/blog/2013/01/09/the-neophytes-guide-to-scala-part-8-welcome-to-the-future.html) and [its sequel](http://danielwestheide.com/blog/2013/01/16/the-neophytes-guide-to-scala-part-9-promises-and-futures-in-practice.html) for some help.
+Make sure that you
+- have only one single blocking call to wait for the result
+- that it is at the very end of your program, and
+- that it has a timeout.
+
+`scala.concurrent.Future` is used in many libraries and the doctor's advice for
+parallel and asynchronous programming in Scala. So it is essential for mastering
+the language and it should become part of your Scala armory.