Add CONTRIBUTING.md

State that info files and documentation describe the software only, and
that core inclusion is decided on licensing, technical, and maintenance
criteria rather than on the views or conduct of core authors.

CONTRIBUTING.md

@@ -0,0 +1,13 @@
+# Contributing to libretro-super
+
+## Core authors and upstream developers
+
+Libretro does not make statements about, endorse, or vet the personal
+views or conduct of core authors or upstream developers.
+
+Info files and other documentation in this repository describe the
+software only. Pull requests that add statements about the people behind
+a core, positive or negative, will not be accepted.
+
+Decisions about including or removing a core are based on licensing,
+technical, and maintenance criteria.