Added GUI usage and tutorial image

README.md

@@ -11,6 +11,18 @@ The following errors will be checked:
 - *NoSubjectError*: Study does not contain any subjects. Mostly because incorrect subject folder or subject prefix selected.
 - *UnknownModalityError*: Modality folder name is not standard.
 
+### GUI Usage
+![](/images/StudyChecker_tutorial.png)
+
+Input fields top down:
+1. _Study Base folder:_
+   Selects the study folder. The study folder is the folder, which contains the subject folders.
+2. _Subject prefix:_
+   If the subject folders of the selected study use a different prefix than "sub-", enter their prefix here.   
+3. _Submit_:
+   Starts performing checks on the given study. 
+
+
 ## License and Citation
 This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License: https://creativecommons.org/licenses/by-sa/4.0/