Added contributor guidelines

Author: etraut-openai

CONTRIBUTING.md

@@ -0,0 +1,8 @@
+# Contributing
+
+Thanks for your interest in contributing! We accept pull requests.
+
+## Bugs and feature requests
+
+Please file all bug reports and feature requests in the
+[openai/codex issue tracker](https://github.com/openai/codex/issues).

README.md

@@ -67,3 +67,7 @@ In addition to the packages specified in the table above, the following packages
 - `elixir`: 1.18.3
 
 See [Dockerfile](Dockerfile) for the full details of installed packages.
+
+## Contributing
+
+See [CONTRIBUTING.md](CONTRIBUTING.md).