dev-release-subproject: Add 'performRelease' setting

When deploying artifacts (i.e. gwtorm, gwtjsonrpc, prolog-cafe) to
the Sonatype repository, it is necessary for the artifacts to be
signed, otherwise releasing will fail.

To sign the artifacts during deployment, 'performRelease' must be
set. Add this to the instructions.

Change-Id: I9eba633d8fdb466ef81823b7132479cfffbb2415
diff --git a/Documentation/dev-release-subproject.txt b/Documentation/dev-release-subproject.txt
index 4886849..c9369b9 100644
--- a/Documentation/dev-release-subproject.txt
+++ b/Documentation/dev-release-subproject.txt
@@ -73,7 +73,7 @@
 * Deploy the new release:
 +
 ----
-  mvn deploy
+  mvn deploy -DperformRelease=true
 ----
 
 * Push the pom change(s) to the project's repository