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

Reviewed-by: sgehwolf, lucy
Backport-of: 22d5e0d1f8849410abe40165b58f45f5e4293884

README.md

-# Welcome to the JDK!
+# Welcome to OpenJDK 17 Updates!
+
+The JDK 17 Updates project uses two GitHub repositories.
+Updates are continuously developed in the repository [jdk17u-dev](https://github.com/openjdk/jdk17u-dev). This is the repository usually targeted by contributors.
+The [jdk17u](https://github.com/openjdk/jdk17u) repository is used for rampdown of the update releases of jdk17u and only accepts critical changes that must make the next release during rampdown. (You probably do not want to target jdk17u).
+
+For more OpenJDK 17 updates specific information such as timelines and contribution guidelines see the [project wiki page](https://wiki.openjdk.org/display/JDKUpdates/JDK+17u/).
+
 
 For build instructions please see the
 [online documentation](https://openjdk.java.net/groups/build/doc/building.html),