Add testing/dev user to the documentation

Help new contributors to hack on pagure by adding one
of the created account to the documentation to avoid to search this
information inside the source code.

Newcomers just find this useful information by reading the documentation.

README.rst

@@ -68,7 +68,9 @@ To populate the container with test data and create a new account, run ::
 
     $ docker-compose -f dev/docker-compose.yml exec web python dev-data.py --all
 
-You can then login with any of the created users.
+You can then login with any of the created users, by example:
+- username: pingou
+- password: testing123
 
 Vagrant
 ^^^^^^^

dev/ansible/roles/pagure-dev/files/motd

@@ -21,4 +21,7 @@ Here are some tips:
 Once you start the server you can navigate to http://localhost:5000/
 in your browser on the host to access your Pagure development environment.
 
+During your navigate you can use any of the created users,
+by example `pingou/testing123`.
+
 Happy hacking!