contribute: add reminder note about docs license (GFDLv1.3+)

Change-Id: Id30cbf3cd37969b46e72e9d4148c91e5a0186424

contribute.rst

@@ -41,6 +41,18 @@ correction.  The patch review process is the same as :doc:`for any
 other Jami project <how-to-submit-a-patch>`, so we will not explain
 every command.
 
+.. note:: By contributing to this documentation, you agree to make
+          your contributions available under the :doc:`fdl`, Version
+          1.3 or any later version published by the Free Software
+          Foundation; with no Invariant Sections, no Front-Cover
+          Texts, and no Back-Cover Texts.
+
+          You are also promising that you are the author of your
+          changes, or that you copied them from a work in the public
+          domain or a work released under a free license that is
+          compatible with the :doc:`fdl`.  DO NOT SUBMIT COPYRIGHTED
+          WORK WITHOUT PERMISSION.
+
 **TODO: internationalization**
 
 Dependencies