many improvements

hosts/yoga-7-pro/README.md

@@ -0,0 +1,14 @@
+Installed using
+```
+> sudo nixos-install --root /tmp/new-install/ --flake .#nyx --impure --no-root-passwd
+```
+
+where `/tmp/new-install` was the moint point of the target FS.
+
+The root user is effectively deactivated, hence the additional setup:
+- users are part of the `wheel` group
+- password needs to be set before first boot:
+    - `nix-enter --root /tmp/new-install`
+    - `passwd <username>`
+
+