docs: compile a list of use cases

Fix #1494

Author: php-coder

docs/use-cases.md

@@ -0,0 +1,12 @@
+# Use Cases
+
+A visitor has one of the following roles:
+- anonymous (non-authenticated)
+- user
+- paid user
+- admin
+
+"Paid user" and "admin" extend "user" role and implicitly have all its permissions.
+
+![A list of use cases](https://github.com/php-coder/mystamps/blob/generated-assets/docs/diagrams/all-use-cases.png)
+