Update README to outline purpose of this repository

[aidanheerdegen]

The README should be updated to explain the purpose of this repository and how it should be used and updated.

This repository is the default location for spack for ACCESS-NRI build and deployment pipelines. It is a maintained fork of the main spack repo: the release branches are maintained by ACCESS-NRI to allow back-porting of bug fixes or updates of important features and/or packages.

[aidanheerdegen]

Also add instructions for how to incorporate upstream commits, including preferred method of formatting commits.

[harshula]

Hi @aidanheerdegen , In the interim, I've added the following note: https://github.com/ACCESS-NRI/spack/?tab=readme-ov-file#pull-requests