Update license part, provide example for derived work.

Author: Kilian

Diff for: CONTRIBUTING.md

@@ -1,28 +1,28 @@
 Contribute to Unknown Horizons!
 ===============================
 
+Folder structure
+----------------
+Create a folder within the root folder with your name/nickname and put all files inside.
+
 License your work
 -----------------
-If you want to upload content, please create a file called LICENSE.md and mention there what is your work and what did you use from other sources and the sources self.
+Please create a file called LICENSE.md within the folder which is named like you/your nickname and mention there what is your work and what did you use from other sources and the sources self.
 An URL should be enough. This should be independant from the license artwork, please mention also Public Domain and Creative Commons Attribution Sharealike 3.0 Unported licensed artwork you used.
 
 Template (use absolute paths):
 ```
 The files:
 
-/nickname/myFile1.png
-/nickname/myFile2.png
-/nickname/myFile3.png
+* /nickname/myFile1.png
+* /nickname/myFile2.png, derived from http://opengameart.org/content/white-bear-low-poly by STKRudy85 licensed as http://creativecommons.org/publicdomain/zero/1.0/
+* /nickname/myFile3.png
 
 are my own work. They are published under the terms of the
 Creative Commons Attribution Sharealike 3.0 Unported License (see CC).
 For more information visit http://creativecommons.org/licenses/by-sa/3.0/legalcode . 
 ```
 
-Folder structure
-----------------
-Create a folder within the root folder with your name/nickname and put all files inside.
-
 Fileformats
 ---------
 Please avoid file formats like .tar, .zip, .xz, .tar.gz or something else which is used as a container for multiple files. One file for everything!