add some information for contributors regarding the copyright header and claify that all contributions are considered AGPLv3 or later

README.md

@@ -30,7 +30,20 @@ if you want to join the Github organization just let us know and we’ll add you
 * ...
 
 ## Contribution Guidelines
-https://nextcloud.com/contribute/
+
+All contributions to this repository from June, 16 2016 on are considered to be
+licensed under the AGPLv3 or any later version.
+
+Nextcloud doesn't require a CLA (Contributor License Agreement).
+The copyright belongs to all the individual contributors. Therefore we recommend
+that every contributor adds following line to the header of a file, if he
+changed it substantially:
+
+```
+@copyright Copyright (c) <year>, <your name> (<your email address>)
+```
+
+More information how to contribute: https://nextcloud.com/contribute/
 
 ## Support
 Learn about the different ways you can get support for Nextcloud: https://nextcloud.com/support/