add more docs about how to locally create the documentation PDF


git-svn-id: https://svn.apache.org/repos/asf/openjpa/trunk@1735191 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/openjpa-project/BUILDING.txt b/openjpa-project/BUILDING.txt
index 248411c..bb89fa2 100644
--- a/openjpa-project/BUILDING.txt
+++ b/openjpa-project/BUILDING.txt
@@ -181,6 +181,13 @@
 
 $ mvn site site:deploy -Pjavadoc-profile,docbook-profile
 
+To generate the docbook documentation you need to cd into the openjpa-project directory and run
+
+$ cd ./openjpa-project
+$ mvn clean install -Papache-release,docbook-profile
+
+The generated PDF is available under ./target/docbook/manual.pdf
+
 
 Running unit tests in the Debugger
 ==================================