docs: Dual license under CC BY-SA and the GFDL

This will allow the text to be used in Wikipedia for example; it
also just makes more sense for documentation than the LGPLv2+.

Closes: #1431

Closes: #1432
Approved by: jlebon

README.md

@@ -108,3 +108,10 @@ Contributing
 ------------
 
 See [Contributing](CONTRIBUTING.md).
+
+
+Licensing
+-------
+
+The license for the *code* of libostree can be found in [COPYING](COPYING).
+The license for the *documentation* of libostree is: `SPDX-License-Identifier: (CC-BY-SA-3.0 OR GFDL-1.3-or-later)`