Add info on custom role key to README doc.

README.md

@@ -17,6 +17,10 @@ LTI_OAUTH_CREDENTIALS = {
 }
 ```
 
-The LTIAuthMiddleware will ensure that all users of your app are authenticated before they can access any page.  Upon successful authentication, a Django user record is created (or updated) and the user is allowed to access the application. 
+* OPTIONALly, you can define a custom role key at the project level. Doing so will cause the middleware to look for any roles associated with that key during the launch request and merge them into the default LTI roles list.  You can declare such a key by adding this to your project configuration:
+```
+LTI_CUSTOM_ROLE_KEY = 'my-custom-role-key-change-me'
+```
 
+The LTIAuthMiddleware will ensure that all users of your app are authenticated before they can access any page.  Upon successful authentication, a Django user record is created (or updated) and the user is allowed to access the application.