docs(contributing): adopt the contributor terms

CONTRIBUTING.md

@@ -1,6 +1,23 @@
 # Contributing
 
-Thanks for contributing to **scripts**.
+Contributions to **scripts** are governed by the Contributor terms below; submitting
+one means you accept them.
+
+## Contributor terms
+
+1. This project belongs to its owner alone. The owner decides what is accepted, in
+   what form and when; the decision is final and needs no justification.
+2. By submitting a contribution you assign to Petr Balvín <opensource@petrbalvin.org>
+   all present and future copyright and related rights in it, worldwide, for the full
+   term of the rights, with the right to relicense and sublicense without restriction,
+   including under proprietary terms.
+3. Where that assignment is not effective, it counts as a perpetual, irrevocable,
+   royalty-free licence with the same scope.
+4. To the fullest extent permitted by law, you waive any right of attribution and
+   integrity in the contribution. The project names no contributors and keeps no
+   credits list.
+5. By submitting you represent that the work is yours and that you hold the rights to
+   assign it as above.
 
 ## Development setup