Rna-transcription: add starter implementation

Complete started implementation added as this is currently the second
exercise in the new ordering.

See issue #178

exercises/rna-transcription/src/main/java/RnaTranscription.java

@@ -0,0 +1,3 @@
+public class RnaTranscription {
+    public String ofDna(String dnaString) { return null; }
+}