added build information

README.md

@@ -8,6 +8,18 @@ This plugin does NOT run an actual migration, it was created to TEST Rosetta's P
 Howto build
 -----------
 
+You can build the plugin by using the "make" tool:
+
+```bash
+# build the plugin JAR file
+$# make build
+
+# clean the build result
+$# make clean
+```
+
+Alternatively, you can also build the plugin using manual steps:
+
 ```bash
 # change to plugin source code directory
 $# cd ~/rosetta_migration_plugin_dummy