add readme for debugging the generator

Author: christothes

codegen/README.MD

@@ -0,0 +1,23 @@
+# Debugging the generator
+
+To configure VS Code for debugging the generator, specifically visitors, add the following to your `launch.json` in the root of the workspace
+
+```json
+{
+    "version": "0.2.0",
+    "configurations": [
+        {
+            "name": "Debug OpenAI Library Plugin",
+            "type": "coreclr",
+            "request": "launch",
+            "program": "dotnet",
+            "args": [
+                "${workspaceFolder}/codegen/dist/generator/Microsoft.TypeSpec.Generator.dll",
+                "${workspaceFolder}",
+                "-g",
+                "OpenAILibraryGenerator"
+            ],
+        }
+    ]
+}
+```