add note about 2.x vs 3.x versions

README.md

@@ -41,7 +41,16 @@ Pact-JVM is written in Scala. As Scala does not provide binary compatibility bet
 artifacts have the version of Scala they were built with in the artifact name. So, for example, the pact-jvm-consumer-junit
 module has a Jar file named pact-jvm-consumer_2.10. The full name of the file is pact-jvm-consumer_2.10-2.0.x.jar.
 
-We currently cross-compile all the artifacts against 2.10 and 2.11 versions of Scala, except for the SBT modules.
+### 3.x branch
+
+3.x branch drops support for JDK 6 and 7 as well as Scala 2.10. It will be cross-compiled with Scala 2.12 when that is
+released, but is currently build against Scala 2.11. All Groovy code is compiled with Invoke Dynamic instruction turned
+on (indy version).
+
+### 2.x branch
+
+2.x versions of pact support JDK 6 and Scala 2.10. We currently cross-compile all the artifacts against 2.10 and
+2.11 versions of Scala, except for the SBT modules.
 
 ## Service Consumers