Add pointer regarding unit tests as inner classes

Author: sopel

rxjava-core/src/test/java/README.txt

@@ -2,4 +2,6 @@ This test folder only contains performance and functional/integration style test
 
 The unit tests themselves are embedded as inner classes of the Java code (such as here https://github.com/Netflix/RxJava/tree/master/rxjava-core/src/main/java/rx/operators).
 
-Also, each of the language adaptors has a /src/test/ folder which further testing. See Groovy for an example: https://github.com/Netflix/RxJava/tree/master/language-adaptors/rxjava-groovy/src/test
+  * For an explanation of this design choice see http://benjchristensen.com/2011/10/23/junit-tests-as-inner-classes/.
+
+Also, each of the language adaptors has a /src/test/ folder which further testing. See Groovy for an example: https://github.com/Netflix/RxJava/tree/master/language-adaptors/rxjava-groovy/src/test