docs: add some links

.github/README.md

@@ -116,6 +116,8 @@ Alternatively, to install this from any NixOS live ISO, run `nix run --experimen
 - These links are your best friends:
   - The nix documentation: https://nix.dev/
   - The nixpkgs reference manual: https://nixos.org/manual/nixpkgs/unstable/#buildpythonapplication-function
+    - the [nixpkgs repository](https://github.com/NixOS/nixpkgs) - especially useful to look at the various READMEs that are in various places in the repository (find using GitHub code search) as well as the Issues and PRs pages
+    - and the [nixpkgs Pull Request Tracker](https://nixpk.gs/pr-tracker.html)
   - The NixOS manual: https://nixos.org/manual/nixos/stable/
   - The NixOS package search: https://search.nixos.org/packages
     - and the nix package version search: https://lazamar.co.uk/nix-versions/