examples: improve Flakes-based VM

- Show login message
- Poweroff on shell exit
- Move to top of examples/README

examples/README.md

@@ -1,8 +1,22 @@
-Examples
----
+## Examples
 
 The easiest way to try out nix-bitcoin is to use one of the provided examples.
 
+### Flakes-based quick start
+
+If you use a Flakes-enabled version of Nix, run the following command to start a minimal
+nix-bitcoin QEMU VM:
+```bash
+nix run github:fort-nix/nix-bitcoin/release
+```
+The VM (defined in [flake.nix](../flake.nix)) runs in the terminal and has `bitcoind`
+and `clightning` installed.\
+It leaves no traces (outside of `/nix/store`) on the host system.
+
+
+### More examples
+
+Clone this repo and enter the examples shell:
 ```bash
 git clone https://github.com/fort-nix/nix-bitcoin
 cd nix-bitcoin/examples/
@@ -79,7 +93,3 @@ The commands in `shell.nix` allow you to locally run the node in a VM or contain
 
 Flakes make it easy to include `nix-bitcoin` in an existing NixOS config.
 The [flakes example](./flakes/flake.nix) shows how to use `nix-bitcoin` as an input to a system flake.
-
-Run `nix run` or `nix run .#vm` from the nix-bitcoin root directory to start an example
-nix-bitcoin node VM.
-This command is defined by the nix-bitcoin flake (in [flake.nix](../flake.nix)).