Added documentation on using multiple databases.

lib/python/Products/ZGadflyDA/README.txt

 ZGadflyDA
 
-
   The ZGadflyDA product provides support for Gadfly database adapter
   objects in the Zope environment.
 
+  Using Multiple Databases with ZGadflyDA
+
+    The ZGadflyDA is intended mainly for demonstration purposes.
+
+    It automatically creates a 'gadfly' directory in your Zope var directory
+    with a 'demo' subdirectory, which provides a demo database.
+
+    To add or use additional databases, simply add (or link) additional
+    directories to your 'var/gadfly' directory.
+