Add hints to hello-world exercise

exercises/hello-world/HINTS.md

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+## Hints
+For this exercise two Scala features come in handy:
+1. [Default Parameter Values](http://docs.scala-lang.org/tutorials/tour/default-parameter-values.html)
+2. [String Interpolation](http://docs.scala-lang.org/overviews/core/string-interpolation.html)
+
+#### Common pitfalls that you should avoid
+- `null` is usually not considered a valid value in Scala, and there are no `null` checks needed (if you don't have to interface with Java code, say). Instead there is the [Option](http://danielwestheide.com/blog/2012/12/19/the-neophytes-guide-to-scala-part-5-the-option-type.html) type if you want to express the possible absence of a value. But for this exercise just assume a normal non-`null` String parameter.
+- Usually there is no need in Scala to use `return`. For a discussion see [here](http://stackoverflow.com/questions/24856106/return-in-a-scala-function-literal). Or as a quote from that discussion: *Don't use return, it makes Scala cry.*