dev: add readme for the dev flake

Author: VTimofeenko

.dev/README.md

@@ -0,0 +1,12 @@
+This is a development subflake of xremap nix flake.
+
+It hosts:
+
+- Formatter that uses `treefmt`
+- Demo nix apps that show how `xremap` service runs in various environments. They
+  are implemented as NixOS virtual machines. Their configs are in `./demo-nixos-configurations/`
+- Module tests in `./checks/`
+
+To run the full test suite, run `nix develop .dev#ci` and execute
+`run-integration-tests`. There may be some evaluation warnings, but they are
+expected.