mention Rationale.md in README.md

Author: samuelgruetter

language-adaptors/rxjava-scala/README.md

@@ -67,6 +67,8 @@ Scala code using Rx should only import members from `rx.lang.scala` and below.
 
 The API documentation can be found [here](http://rxscala.github.io/scaladoc/index.html#rx.lang.scala.Observable).
 
+Note that starting from version 0.15, `rx.lang.scala.Observable` is not a value class any more.  [./Rationale.md](https://github.com/Netflix/RxJava/blob/master/language-adaptors/rxjava-scala/Rationale.md) explains why.
+
 You can build the API documentation yourself by running `./gradlew scaladoc` in the RxJava root directory.
 
 Then navigate to `RxJava/language-adaptors/rxjava-scala/build/docs/scaladoc/index.html` to display it.