Add Nix/NixOS installation section to README

Author: onsails

README.md

@@ -20,6 +20,18 @@ cargo install ccsync
 curl https://i.jpillora.com/onsails/ccsync@ccsync-v0.1.5! | bash
 ```
 
+### Using Nix/NixOS
+
+> **Note:** NixOS package is pending review ([PR #456721](https://github.com/NixOS/nixpkgs/pull/456721))
+
+```bash
+# Once merged, install with:
+nix profile install nixpkgs#ccsync
+
+# Or use in a shell:
+nix shell nixpkgs#ccsync
+```
+
 ## 🚀 Quick Start
 
 ```bash