add site generation command (which is not so easy to know)

Author: hboutemy

README.md

@@ -11,7 +11,13 @@ Modello generates code from a simple model format: based on a plugin architectur
 
 # Documentation
 
-Documentation can be found on the Modello site hosted at [Github.io](http://codehaus-plexus.github.io/modello/)
+Documentation can be found on the Modello site hosted at [Github.io](http://codehaus-plexus.github.io/modello/).
+
+This site is generated by running:
+
+```
+mvn -Prun-its,reporting clean verify site site:stage
+```
 
 # Licensing