instruction for metamodel generation tool

git-svn-id: https://svn.apache.org/repos/asf/openjpa/trunk@793610 13f79535-47bb-0310-9956-ffa450edef68

scripts/mmg.readme.txt

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+Metamodel generator (mmg)
+
+JPA 2.0 provides an utility to generate canonical metamodel for each persistent domain entity.
+This brief note describes how to generate these canonical metamodel classes.
+
+The canonical metamodel classes are generated during compilation of persistent domain entity classes.
+The generation process depends on Annotation Processing facility available in JDK6. Naturally, this
+process *only* works if the persistent domain entity classes are compiled with a JDK6 compiler.
+
+Effectively, the process appears to be normal compilation process with a specialized Annotation Processor
+
+  $ javac -classpath %CLASSPATH% <javac options> <mmg options> <*.java source files>
+  
+  
+Compiler Classpath
+------------------
+   The compiler classpath must include OpenJPA libraries. 
+
+mmg options
+-----------
+   These options specify the Annotation processor to be used as well as options for the processor. The only mandatory 
+   option is 
+          -processor org.apache.openjpa.persistence.meta.AnnotationProcessor6
+          
+The other options qualify the  Annotation processor itself and specified with -A<option>=<value> syntax
+
+-Alog     log level. Permissible values are TRACE, INFO and WARN. 
+          Default is INFO.
+-Aout     the directory location of the generated canonical model source output files. 
+          Default is the current directory.
+-Asource  compatibility level of the generated canonical model source code. Permissible values are 5 and 6. 
+          Default is 6.
+-Aheader  flags if a header is created for the generated source code. Permissible values are true and false. 
+          Default is true.
+