fleshed out the setup guide as it was lacking

[kamisori]

Had trouble with original setup instructions, these are the steps that got me to a working ArcadiaGodot project

[selfsame]

Thank you for the heads up on the setup issues! These have been added to the readme in https://github.com/arcadia-unity/ArcadiaGodot/commit/9c1daf713eb651e267c1cafebbf772735455bfe0