feat(usage): add basic usage-guide

docs/usage.md

 # Usage
 
-!!! todo
-    Write usage guide for lecturers and students
+On this page you'll find all information regarding the usage of Dockerized Testing Toolkit for lecturers as-well as students.
+
+## Lecturer
+
+### Requirements
+
+- testrunner docker-image hosted on DockerHub. Either:
+    1. choose from the official supported list
+    2. find/create custom image
+- tests code base in internet accessible Git-Repo
+- file-structure of code-base to match choosen runner's specifications
+
+### Create/Edit Assignment in Moodle
+
+In your Moodle course
+
+- add a new Assignment
+- enable Dockerized Testing Toolkit
+- disable all other submission types
+- upload textfile with your assignment settings (see below on file specification)
+
+To edit an existing assignment, just remove the old and upload a new config file.
+## Student
+
+As student you can either zip your submission, or upload a textfile with the link to your git repository.
+For the file specifications see below.
+## DTT configuration file specification
+
+This section describes how the DTT-config-files work.
+
+They have to contain an URI starting with `dtt` as identifier, then followed by the information as shown below.
+### Lecturer (assignment)
+
+Information needed in the config-file:
+
+- url to git repo
+- user and password, if needed for access
+- docker image to use
+
+```
+dtt::<git-repo-url>::<none|<user name>>::<none|<user name>>::<docker image[:tag]>
+```
+
+Example:
+
+```
+# test code
+dtt::https://transfer.hft-stuttgart.de/gitlab/dtt/example_openjdk11-junit5-calculator-test.git::none::none::hftstuttgart/dtt-openjdk11-junit5-testrunner:feat-dtt-transition
+```
+
+### Student (submission)
+
+Information needed in the config-file:
+
+- url to git repo
+- user and password, if needed for access  
+
+```
+dtt::<git-repo-url>::<none|<user name>>::<none|<user name>>
+```
+
+!!! tip
+    Additionally any number of [Unified-Ticketing][1] URIs can be in the config file for ticket response.
+
+Example:
+
+```
+# source code
+dtt::https://transfer.hft-stuttgart.de/gitlab/dtt/example_openjdk11-junit5-calculator-submission.git::none::none
+
+# disabled, just as example
+# ticket response
+# unifiedticketing:gitlab:https://transfer.hft-stuttgart.de/gitlab:0815:ABASAEUIBFI44BKW34U7I24
+```
+
+<!-- Unified-Ticketing -->
+[1]: https://transfer.hft-stuttgart.de/pages/9lukas5/unified-ticketing/api/core/ticket-system.html#by-uri