Add documentation

Author: nicolasstucki

compiler/test/dotty/tools/vulpix/TestGroup.scala

@@ -1,5 +1,11 @@
 package dotty.tools.vulpix
 
+/** Test groups are used to ensure that the ouput of tests do not overlap.
+ * 
+ *  It can be used to disambiguate ouputs of tests that test the same file but with different options as shown in the following example.
+ *    compileFilesInDir("../tests/pos", defaultOptions)(TestGroup("compileStdLib")) // will output in ./out/compileStdLib/...
+ *    compileFilesInDir("../tests/pos", defaultOptimised)(TestGroup("optimised/testOptimised")) // will output in ./out/optimised/testOptimised/...
+ */
 class TestGroup(val name: String) extends AnyVal {
   override def toString: String = name
 }