Added section on testing API with Postman

Included link to external pre-prepared Postman requests

Author: Sam Tuke

README.md

@@ -36,6 +36,12 @@ Please install this package via Composer from within the
 [phpList base distribution](https://github.com/phpList/base-distribution),
 which also has more detailed installation instructions in the README.
 
+## Local demo with Postman
+
+You can try out the API using pre-prepared requests and the Postman GUI 
+tool. Install Postman as a browser extension or stand-alone app, open the 
+[phpList 4 REST API Demo collection](https://documenter.getpostman.com/view/3293511/phplist-4-rest-api-demo/RVftkC9t#4710e871-973d-46fa-94b7-727fdc292cd5) and click 'Run in Postman'.
+
 
 ## Contributing to this package