Add use case diagram

Rewritten-docs.md

@@ -148,15 +148,10 @@ points, a configuration for calculating the final score, a maximum number of
 submissions, and a list of supported runtime environemnts (e.g., programming
 languages) including specific time and memory limits for the sandboxed tasks.
 
-Typical use cases for supported user roles are ilustrated on following picture:
+Typical use cases for supported user roles are ilustrated on following UML
+diagram:
 
-@todo: UML use case diagram (and improve or delete following paragraph)
+![System use case diagram](https://github.com/ReCodEx/wiki/raw/master/images/System_use_case.png)
-
-Priviledged user (supervisor) can create exercise, assign it in his/her group,
-change assignment details, view results of his/her students and manually alter
-them. Normal user (student) can join a group, get list of assigned exercises,
-view assignment detail, submit his/her solution and view the results of the
-evaluation.
 
 #### Exercise evaluation chain