Updated readme with more info on props

README.md

@@ -97,6 +97,18 @@ To evaluate props when building the archive:
 
     props exec="generate-props.php";
 
+The props are defined, and the keys are cast to upper case. Be careful if you
+attempt to redefine any of these values from within your project as this will
+fail. What you should do is something like:
+
+    if (!defined('MYAPP_VERSION')) define('MYAPP_VERSION','dev');
+    echo "This is MyApp ".MYAPP_VERSION."\n";
+
+This makes sure that you don't try to set an existing define, and also that the
+defines generated from props by MakePhar is also available when running the app
+from source.
+
+
 ## Options and Tweaks
 
 Boolean options (such as `verbatim` and `library`) will have their value default