Add copilot-instructions.md file

Author: AArnott

.github/copilot-instructions.md

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+# Copilot instructions for this repository
+
+## High level guidance
+
+* Review the `CONTRIBUTING.md` file for instructions to build and test the software.
+* Set the `NBGV_GitEngine` environment variable to `Disabled` before running any `dotnet` or `msbuild` commands.
+
+## Software Design
+
+* Design APIs to be highly testable, and all functionality should be tested.
+* Avoid introducing binary breaking changes in public APIs of projects under `src` unless their project files have `IsPackable` set to `false`.
+
+## Testing
+
+* There should generally be one test project (under the `test` directory) per shipping project (under the `src` directory). Test projects are named after the project being tested with a `.Test` suffix.
+* Tests should use the Xunit testing framework.
+
+## Coding style
+
+* Honor StyleCop rules and fix any reported build warnings *after* getting tests to pass.
+* In C# files, use namespace *statements* instead of namespace *blocks* for all new files.
+* Add API doc comments to all new public and internal members.