Add document for .chat folder in repository root (vscode)

[narrowizard]

Is your feature request related to a problem? Please describe. The auto generated folder .chat seems saving some predefined prompt and command, but I am not sure how to extend it properly. Though I can fork one from exist ones, maybe it's not exactly correct usage.

Describe the solution you'd like I want a document to tell me what responsiblity each file take, and what format should it be. Also what key value each config file contains.