Add hints to robot-name exercise

Author: abo64

exercises/robot-name/HINTS.md

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+## Hints
+Make sure your solution is general enough to be easily scalable for longer names containing more letters and digits. This usually makes for better code quality, too.
+
+Suggestion (this is not explicitly tested):
+To make sure you always have a unique name you could implement your own cache cache or use a `Stream` with its built-in cache.