Add paragraph about custom libraries

Author: fwsGonzo

docs/godot_docs/libraries.md

@@ -6,6 +6,8 @@ sidebar_position: 11
 
 Libraries make it easy to use pre-made programs in the sandbox.
 
+Here is a complete working example for adding sandboxed LuaJIT to your project:
+
 ```py
     if !FileAccess.file_exists("res://luajit.elf"):
         var buffer = Sandbox.download_program("luajit")
@@ -38,3 +40,13 @@ Libraries make it easy to use pre-made programs in the sandbox.
 There is [a repository with pre-made programs](https://github.com/libriscv/godot-sandbox-programs).
 
 You can find [the latest release here](https://github.com/libriscv/godot-sandbox-programs/releases/latest). The latest release is used by `Sandbox.download_program(name)`.
+
+## Custom libraries
+
+You can provide your own library, but it must be added to the project in project settings. Under Editor -> Script -> Program Libraries you can add a new Dictionary entry with key `library_name`and value `github_user/repository_name`. Once added, `library_name/artifact` will now refer to `https://github.com/github_user/repository_name/releases/latest/artifact.zip`. That is:
+
+```py
+var buffer = Sandbox.download_program("library_name/artifact")
+```
+
+Will download the URL mentioned above.