Add instructions for copilot

Author: pascalberger

.github/copilot-instructions.md

@@ -0,0 +1,35 @@
+This is a .NET based repository containing recipes for the Cake Build Automation System bundling individual Cake Issues addins.
+The individual recipes are published as NuGet packages.
+
+See https://cakeissues.net/ for documentation and more information about Cake Issues.
+
+Follow these guidelines when contributing:
+
+## Before committing code
+- Ensure no warning or error messages from Roslyn analyzers are present in the code.
+- Ensure Unit Tests are passing.
+- Ensure Integration Tests are passing.
+
+## Development Flow
+- Only building: `build.sh --target=DotNetCore-Build`
+- Publish NuGet Packages: `build.sh --target=Create-NuGet-Packages`
+- Run Unit Tests: `build.sh --target=Test`
+- Run Integration Tests:
+  - Run first `build.sh --target=Create-NuGet-Packages` in the root directory to create the NuGet packages.
+  - Run `build.sh` in the `tests/<RECIPE-NAME>/<TEST>` directory to run the integration tests for the corresponding recipe
+- Full CI check: `build.sh` (includes build, publish, test)
+
+To have verbose output of build.sh the following parameter can be add `--verbosity=diagnostic`
+
+## Repository Structure
+- `Cake.Frosting.Issues.Recipe`: Contains the source code for the recipe for use with Cake Frosting
+- `Cake.Issues.Recipe`: Contains the source code for the recipe for use with Cake Scripting
+- `nuspec/nuget`: Contains the NuGet specification files for the Cake Scripting addin
+- `tests`: Contains integration tests for the recipes
+- `.github/workflows`: Contains GitHub Actions workflows for CI/CD which need to be maintained
+- `.azuredevops/pipelines/templates`: Contains Azure Pipelines templates which need to be maintained. The main file is `azure-pipelines.yml` in the root directory.
+
+## Key Guidelines
+1. Maintain existing code structure and organization
+2. Write unit tests for new functionality.
+3. Write integration tests for new functionality which can't be tested with unit tests.