Add GitHub Copilot config, limiting it to managed code

Crucially, having it self-identify on every change, and leave comments.
I'll look into other agents later; there's no standard for how to
configure them like there is for IDEs, but on the bright side the config
is all natural language so a symlink to this file would probably work.

Author: YoshiRulz

.github/copilot-instructions.md

@@ -0,0 +1,25 @@
+# Project Overview
+
+This is a retro game emulator project, though emulation is mostly handled by libraries, while this repo is focused on the frontend.
+
+## Folder Structure
+
+- `/ExternalProjects`: Contains the source code for some vendored C# libraries.
+- `/src`: Contains the source code for the libraries and the frontend.
+
+Only change C# source files (`*.cs`) and project files (`*.csproj`, `*.props`).
+
+## Libraries and Frameworks
+
+- .NET Framework 4.8 for the frontend.
+- .NET Standard 2.0 for the libraries, though some newer features are available thanks to polyfills.
+
+## Coding Standards
+
+- Follow the code style conventions given in the root EditorConfig file.
+- Leave a comment beside each block of code you touch with your name/version and an explanation of how the old code was broken.
+
+## UI guidelines
+
+- Use American English spellings in strings.
+- Avoid changing the GUI if possible. The GUI encompasses instances of the `Form` and `Control` classes from Windows Forms, which usually have a partial class definition in a file named `*.Designer.cs`.