add reference for bash script

README.md

 ## Extract assignments
 This bash script is created to extract files from blackboard. Blackboard has a
 horrible way of creating a zipfile. This tries to fix it.
+
+## References
+This guy helped me in making the bashscript useable for other people:
+http://tuxtweaks.com/2014/05/bash-getopts/
+
+You'll notice that some of the code is directly copied, but also altered to fit
+my needs.
+