Add Copilot instructions

Author: hilli

.github/copilot-instructions.md

@@ -0,0 +1,14 @@
+We strongly prefer that Go tests are written in a way that they can be run with `go test` without any additional setup. This means that tests should not require any external files or resources, and should be self-contained.
+
+Adher to the specification by running the following command to test the implementation against the Cooklang specification:
+
+```shell
+task test-spec
+```
+
+```shell
+task test # Run all tests
+go test -v ./... # Run all tests with verbose output
+```
+
+We are using a [taskfile](../Taskfile.yaml) as a makefile replacement to execute tests.