feat(doc): provide nix installation method

Add documentation for installing using Nix, including instructions specific to NixOS.
See: NixOS/nixpkgs#366586

The pr has been merged and should be available within a very day.

README.md

@@ -18,6 +18,14 @@ Unlike other tools, rwalk does **<u>not</u>** provide advanced fuzzing features
 
 ## Quick Installation
 
+### On [Nix](https://nixos.org)
+```bash
+# without flakes:
+nix-env -iA nixpkgs.rwalk
+# with flakes:
+nix profile install nixpkgs#rwalk
+```
+
 ### From [homebrew](https://brew.sh) <!-- omit in toc -->
 
 ```bash