[helios] add Helios-specific README

README.md

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+# Helios fork of [omniosorg/omnios-build](https://github.com/omniosorg/omnios-build) (Build System for OmniOS)
+
+This repository contains build scripts derived from OmniOS.  Packages for
+Helios 2 are based on upstream OmniOS r151046 LTS at this time, with local
+Helios-specific changes as required.
+
+See also: [`README.upstream.md`](./README.upstream.md).
+
+## Licence
+
+Unless otherwise noted, local changes are supplied under the terms of the
+[Common Development and Distribution License ("CDDL"), version 1.0](./LICENSE)
+for compatibility with the upstream OmniOS project.  You may only use these
+files in accordance with the terms of version 1.0 of the CDDL.
+
+Copyright 2024 Oxide Computer Company