vscode-ext: Document how to access remote repositories by branch

[ChristopherHX]

The following seems to work for GitHub Remotes, if you connect to a remote repository it uses a sha instead of a readable name...
await code.workspace.fs.readDirectory(code.Uri.parse("vscode-vfs://github%2Bmain/christopherhx/github-act-runner")).

refs with a / seem to be able to use percent encoding in the authority segment