LICENSE: add a note about data/ licensing for future added data.

I am adding this note as a consequence of this discussion on Pixls.us:
https://discuss.pixls.us/t/gimp-patterns-and-the-gpl-license/35969

Now we were always very clear that GIMP can be used for whatever use you want
and that the project doesn't intent to claim any rights on produced artworks.

There is even a FAQ entry about the fact we don't intend to put any restriction
on people's work (though this FAQ entry was added in 2015 AFAICS):
https://www.gimp.org/docs/userfaq.html#can-i-use-gimp-commercially

Yet it's better to be as clear as possible. Therefore this new note in the
LICENSE file will serve to make sure any new data is properly licensed CC0 by
the simple fact of contributing it.
This is similar to the fact that people contributing patches to core GIMP (app/)
are implicitly licensing it as GPL. Now people contributing data to data/ are
implicitly licensing it as CC0 as per our LICENSE file.

LICENSE

@@ -27,3 +27,23 @@
 
 * Icon themes are licensed under Creative Commons by-sa 3.0 or 4.0. See
   the 'COPYING' files in icons/Symbolic and icons/Color respectively.
+
+* Any data used in artworks, such as brushes, patterns and the like, and more
+  broadly likely all data inside the data/ folder should be under a CC0 license
+  (Creative Commons Zero) or equivalent, i.e. "No rights Reserved", because GIMP
+  contributors clearly don't intend to claim any right on anyone's work just
+  because they used GIMP and its default data.
+  We cannot clearly give proper licensing on all existing data, prior to the
+  addition of this note in the LICENSE file (for historical reasons, simply
+  because their contributors are since long gone), though we can say that an
+  uncountable number of users have used these for dozens of years and never had
+  legal problems as far as we know (it is anyway unclear whether anyone could
+  really claim any right for very basic brush or pattern usage). Since 2015, we
+  even have a clear FAQ entry to explicitly say the GIMP project has no
+  intention whatsoever to put restrictions on people's work:
+  https://www.gimp.org/docs/userfaq.html#can-i-use-gimp-commercially
+
+  Therefore any new data file under the data/ folder will be expected to be CC0.
+  All contributors are expected to read the LICENSE file and therefore are
+  implicitly agreeing to license their data under CC0 by contributing it as core
+  GIMP data.