8343474: [updates] Customize README.md to specifics of update project

Reviewed-by: sgehwolf
Backport-of: ac1f868

Author: vieiro

README.md

@@ -1,4 +1,11 @@
-# Welcome to the JDK!
+# Welcome to OpenJDK 11 Updates!
+
+The JDK 11 Updates project uses two GitHub repositories.
+Updates are continuously developed in the repository [jdk11u-dev](https://github.com/openjdk/jdk11u-dev). This is the repository usually targeted by contributors.
+The [jdk11u](https://github.com/openjdk/jdk11u) repository is used for rampdown of the update releases of jdk11u and only accepts critical changes that must make the next release during rampdown. (You probably do not want to target jdk11u).
+
+For more OpenJDK 11 updates specific information such as timelines and contribution guidelines see the [project wiki page](https://wiki.openjdk.org/display/JDKUpdates/JDK11u).
+
 
 For build instructions please see the
 [online documentation](https://openjdk.java.net/groups/build/doc/building.html),