README.md in mcfly-0.0.4 vs README.md in mcfly-0.0.5

@@ -87,12 +87,17 @@
 
 ## Implementation
 
 TODO
 
-## Limitations
+## Limitations/Requirements
 
-Currently, Mcfly only works with PostgreSQL databases.
+Currently, Mcfly only works with PostgreSQL databases. The following
+line must be added to the `postgresql.conf` file.  Mcfly uses the
+PostgreSQL session variable `mcfly.whodunnit` to store the current
+user id.
+
+    custom_variable_classes = 'mcfly'
 
 ## History
 
 The database table versioning mechanism used in Mcfly was originally
 developed at [TWINSUN][]. It has since been modified and enhanced at