Explain policy about contributing other code; closes greghendershott#720

CONTRIBUTING.org

@@ -40,6 +40,22 @@ both:
 1. the =Package-Requires:= line in =racket-mode.el=
 2. the =deps= target in =Makefile=
 
+** Contributing code you did not write yourself
+
+It is fine to propose adding code that you copied from elsewhere,
+provided you say where ("provenance") and the license (e.g. "GPL",
+"MIT", etc.). Including a URL in a source code comment is ideal.
+
+As a GPL project, we can use code from most other types of licenses,
+but we need to know exactly which license, if any.
+
+Also we prefer to give credit ("attribution"), and in fact some
+licenses require this.
+
+**Important**: Because it is impossible to know the provenance/license
+of code generated by an LLM or "AI" (such as GitHub Copilot) we cannot
+accept such code.
+
 ** Pointing Emacs to your Git clone
 
 After installing dependencies you should just need to add the path to