Update README to be consistent with godot's

[Mobilpadde]

Description

Could you please update the README to be in consistence with http://docs.godotengine.org/en/3.0/tutorials/plugins/gdnative/gdnative-c-example.html ?

Asking because I can simply not figure how to use this? 😢

Details

Go version	1.10.3
Godot version	v3.0.6.stable
Godot-Go version/commit hash	1746202a2001f65ff7eb635b8cd7fc2a6ea64ca2
OS	elementary OS 0.4.1 Loki