docs(README): use $RANDOM to allow for a probably unique username 😉 (#506)

Author: agconti

README.md

@@ -51,7 +51,7 @@ Example of the result: https://github.com/agconti/piedpiper-web
 Try creating a user!
 
 ```bash
-curl -d '{"username":"testuser", "password":"test", "email":"test@test.com", "first_name":"test", "last_name":"user"}' \
+curl -d '{"username":"'"$RANDOM"'", "password":"test", "email":"test@test.com", "first_name":"test", "last_name":"user"}' \
      -H "Content-Type: application/json" \
      -X POST https://piedpiper-prod.herokuapp.com/api/v1/users/
 ```