Add a how-to-translate guide.

[Gulluth]

I want to encourage translations for the Vaarn SRD, but I also want to create a process that can be followed so the translation repositories end up correctly formatted for inclusion in the language selector.

There is a Spanish translation provided by Inguaz and Dark Soups on the VoV Discord that I will use to define this process as I convert the google doc to web.

I have a rough document started by google gemini that defines this process very generically, I will use this as a starting point but it needs significant modification. Thanks evil AI overlords for the first draft!