Add README.md with some introduction

Author: akien-mga

README.md

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+# Godot in-container build scripts
+
+Build scripts used for official Godot Engine releases using containers
+built from https://github.com/godotengine/build-containers
+
+## Disclaimer
+
+This repository is **not** intended for end users, and thus not
+supported. It's only public as a way to document our build workflow,
+and for anyone to use as reference for their own buildsystems.
+
+We will eventually release a public build script that integrates all
+this in a simple and user-friendly interface.
+
+## Usage
+
+- Build containers using https://github.com/godotengine/build-containers
+- Copy `config.sh.in` as `config.sh` and configure it as you want.
+- Edit `build.sh` to properly reference those containers if local, or
+  use `config.sh` to point to your own registry if you uploaded
+  containers.
+- Build with `build.sh` (check `--help` for usage).
+- Package binaries with `build-release.sh` (check `--help` for usage).
+- Build templates .tpz with `build-templates.sh` (check `--help` for
+  usage).
+
+Example that builds Godot 3.2-stable Classical (not Mono):
+```
+./build.sh -v 3.2-stable -g 3.2-stable -b classical
+./build-release.sh -v 3.2-stable -b classical
+./build-templates.sh -v 3.2-stable -t 3.2.stable -b classical
+```
+
+Again, this is intended for release managers and usability is not the
+main focus. Tweak the build scripts to match your own requirements if
+you want to use this until we provide a better, user-friendly
+interface.