Add tips

tips/test_on_vm.md

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+## Configure VM
+
+Configure VM
+```nix
+users.users.<user>.initialPassword = "<password>";
+virtualisation.vmVariant = {
+  # following configuration is added only when building VM with build-vm
+  virtualisation = {
+    memorySize = <RAM in MiB>; # Use 8192MiB memory.
+    cores = <CPU Core number>;
+    # And more here https://github.com/NixOS/nixpkgs/blob/nixos-unstable/nixos/modules/virtualisation/qemu-vm.nix
+  };
+};
+```
+
+Build
+```bash
+nixos-rebuild build-vm --flake .#nixos-test
+```
+
+Run
+```bash
+./result/bin/run-nixos-vm-vm
+```