add a default readme.java.md in samplereadme (#28338)

documentation/samplefiles/readme.java.md

@@ -0,0 +1,7 @@
+## Java
+
+These settings apply only when `--java` is specified on the command line.
+
+``` yaml $(java)
+client-flattened-annotation-target: disabled
+```

documentation/samplefiles/samplereadme.md

@@ -76,3 +76,7 @@ See configuration in [readme.typescript.md](./readme.typescript.md)
 ## CSharp
 
 See configuration in [readme.csharp.md](./readme.csharp.md)
+
+## Java
+
+See configuration in [readme.java.md](./readme.java.md)