Localizing extensions guide

[KamasamaK]

Now that extension localization is built into the core of VS Code, it would be nice to have a proper localization guide for extensions.

[gregvanl]

@TylerLeonhardt

[TylerLeonhardt]

Agreed! You can find a good overview over in https://github.com/Microsoft/vscode-l10n but a guide on the website would be great