Add hints to phone-number exercise

exercises/phone-number/HINTS.md

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+## Hints
+For simplicity and readability: Consider using the Scala collection functions instead of Java's `String` methods. Remember that in Scala is implicitly also a `Seq[Char]`, so you can call them as easily as the `String` methods.
+Some examples:
+- `filter` instead of `replaceAll`
+- `take`, `takeRight`, `drop`, `head`, `tail` instead of `substring`
+
+Another idea worth exploring might be to change the `String` into a `List[Char]`
+and then use [pattern matching](http://alvinalexander.com/scala/how-to-use-lists-nil-cons-scala-match-case-expressions) with the `::` operator.