docs: add development

DEVELOPMENT.md

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+# Development
+
+## Setup
+
+You need incus installed
+
+Following nixos config is recommended:
+
+```nix
+{
+  virtualisation.incus.enable = true;
+  networking.nftables.enable = true;
+  networking.firewall.trustedInterfaces = [ "incusbr0" "incusbr1" ];
+  users.users.YOU.extraGroups = [ "incus-admin" ];
+}
+```
+
+Then you can init incus: `sudo incus admin init --minimal`
+
+## Testing
+
+- `bash test/incus-test-setup.sh`: Create containers for testing
+- `bash test/test-deployment.sh`: Deploy example.nix on all containers
+
+## Automated tests
+
+If you want to run the full testsuite first run `EXPORT_IMAGE=1 bash test/incus-test-setup.sh` to create images from instances then `bash test/test-all.sh` to run the testsuite