Add the ending Java 6 support notice to README (#483)

Author: chingor13

README.md

@@ -47,3 +47,16 @@ Java 10 | [![Kokoro CI](https://storage.googleapis.com/cloud-devrel-public/java/
 ## Links
 
 - [Discuss](https://groups.google.com/group/google-http-java-client)
+
+## Notice: Ending Java 6 Support
+
+Please note: since Java 6 extended support is being ended this December by Oracle, we will begin
+ending Java 6 support in early 2019, with release 1.28.0 as a tentative goal. Users may stay still
+use these libraries in Java 6 projects for some time, but going forward we will not ensure that
+these libraries work in such an environment. After 1.28.0, our supported versions will include Java
+7 and onward.
+
+For Android users, we will continue our 4.0 support.
+
+For questions or concerns, please file an issue in the GitHub repository.
+