Merge pull request #3998 from dotty-staging/doc/java-not-yet-9

Clarify we do NOT yet support Java 9.

Author: Blaisorblade

docs/docs/contributing/getting-started.md

@@ -7,8 +7,9 @@ title: Getting Started
 
 Requirements
 ------------
-Make sure that you are using Java 8 or later. The output of `java -version`
-should contain `1.8`.
+Make sure that you are using Java 8: The output of `java -version` should
+contain `1.8`. Work to support Java 9 is
+[ongoing](https://github.com/lampepfl/dotty/pull/3138).
 
 Compiling and Running
 ---------------------