rna-transcription: explain UnsupportedOperationException in HINT.md file

exercises/rna-transcription/HINT.md

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+The starter implementation for this exercise uses [UnsupportedOperationException](http://docs.oracle.com/javase/8/docs/api/?java/lang/UnsupportedOperationException.html). 
+This [exception](https://docs.oracle.com/javase/tutorial/essential/exceptions/) will cause the test to fail with the provided error message. 
+
+To make the test pass you need to remove the line which throws the exception and replace it with your implementation of the method.