FeatureRequest: Update README with instructions to use GDExtension without prior knowledge of GDNative

[gitatr]

The current README.md instructions assume knowledge of the workflow for Godot 3.x GDNative.
It would be nice to have a guide to use GDExtension from scratch. Without prior knowledge of GDNative.

References like this for example:

Compiling this repository generates a static library to be linked with your shared lib, just like before.

... are not immediately useful.

[dsge]

References like this for example:

Compiling this repository generates a static library to be linked with your shared lib, just like before.

... are not immediately useful.

That part could be renamed from "Getting started" to "Upgrading from Godot 3.x GDNative".

A brand new "Getting started" section, in addition to instructions on how to get started from scratch, could also have examples on

• writing _ready() and _process() functions,
• using get_node(),
• correctly instantiating custom/builtin nodes,
• loading other scenes at runtime

All of these are missing even from the demo project included in this repo, yet I assume they would be quite commonly used in practice.

[the-argus]

I am trying to build a game pretty much entirely in C++, but I have no prior knowledge of doing so in GDNative. I'm now prepared to go searching through the headers the source files to figure this out. I'll try to contribute to the documentation as I go but would appreciate if someone who already knows the codebase could get some real documentation going. More than just a README would be amazing, too: readthedocs.io like the rest of godot.