docs/flake: document how to build a pull request

It's not so common knowledge that forges also expose pull requests as
git refs. But it's actually a cool way of quickly testing someones
contribution, so I found it worth specifically mentioning it.

src/nix/flake.md

@@ -84,6 +84,8 @@ Here are some examples of flake references in their URL-like representation:
   repository on GitHub.
 * `github:NixOS/nixpkgs/nixos-20.09`: The `nixos-20.09` branch of the
   `nixpkgs` repository.
+* `github:NixOS/nixpkgs/pull/357207/head`: The `357207` pull request
+   of the nixpkgs repository.
 * `github:NixOS/nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293`: A
   specific revision of the `nixpkgs` repository.
 * `github:edolstra/nix-warez?dir=blender`: A flake in a subdirectory