add Readme with build and test instructions

Readme.md

@@ -0,0 +1,10 @@
+## NixOS overlay for [Seafile](https://seafile.com)
+
+This is an ongoing attempt to package the Seafile server and create a NixOS module for it.
+
+### testing
+
+- building package:
+  1. add this overlay to your [nixpkgs overlay configuration](https://nixos.wiki/wiki/Overlays)
+  1. `nix-build '<nixpkgs>' -A seafile-server`
+- testing the NixOS module in a VM: `NIXOS_CONFIG="$(pwd)/seafile-test.nix" nixos-rebuild build-vm --fast --show-trace && result/bin/run-nixos-vm`