docs: add CLI build and run instructions to README

Author: drewr

README.md

@@ -29,9 +29,30 @@ cd ui
 dx serve --platform desktop
 ```
 
+**Build the CLI:**
+
+With Cargo:
+```bash
+cd cli
+cargo build --release
+./target/release/datum-connect --help
+```
+
+With Nix:
+```bash
+nix build .#cli
+./result/bin/datum-connect --help
+```
+
 **Run the CLI:**
 
+With Cargo:
 ```bash
 cd cli
-cargo run -- --help
+cargo run -- tunnel --help
+```
+
+With Nix:
+```bash
+nix run .#cli -- tunnel --help
 ```