Add reserved namespaces policy

[rubenwardy]

Fixes #222

[MrRar]

What's so bad about using a game prefix? Maybe someone wants to make a mod that only applies to a specific game.

[rollerozxa]

There's no official reserved namespace policy set into stone yet, hence why this is an open, unmerged pull request. Right now each game on ContentDB has its namespace implicitly reserved and mods under that namespace should either be part of the game or mods that are specifically intended for that game.