Adds notice about need to add key config for default patron branch ID.

README.md

@@ -8,6 +8,7 @@ Remember to add the following keys to your site in order for the module to work:
 * POLARIS_API_STAFF_USERNAME
 * POLARIS_API_STAFF_PASSWORD
 * POLARIS_API_WORKSTATION_ID
+* POLARIS_API_DEFAULT_PATRON_BRANCH_ID
 
 TODO:
 - Polaris core client in src/RCPL should be added as a new project on packagist