Add additional information to README

Provide a reference to the rendered documentation, CI status, and
releases. Update the status section to reflect things as they are.

README.md

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+[![Build Status](https://github.com/FirmwareHandoff/firmware_handoff/actions/workflows/main.yml/badge.svg)](https://github.com/FirmwareHandoff/firmware_handoff/actions/workflows/main.yml)
+[![Release Version](https://img.shields.io/github/v/release/FirmwareHandoff/firmware_handoff?label=release)](https://github.com/FirmwareHandoff/firmware_handoff/releases)
+
+This repository contains the Firmware Handoff specification, which defines a
+data structure to transfer essential configuration information between firmware
+stages during platform initialization.
+
+The documentation is generated using the Sphinx framework. A version of this
+specification, rendered in HTML, is available
+[here](https://firmwarehandoff.github.io/firmware_handoff/).
+
+Project dependencies
+====================
+
+For an Ubuntu development machine, install the following packages to build the specification:
+
+- `librsvg2-bin`
+- `python3-sphinx`
+- `python3-sphinxcontrib.svg2pdfconverter`
+- `python3-sphinx-rtd-theme`
+- `latexmk`
+- `texlive-latex-extra`
+
+**Note:** This list has been tested on Ubuntu 20.04 LTS and 22.04 LTS running on AArch64 and AMD64.
+
+Building the document
+=====================
+
+The following are use to generate the specification:
+
+- pdf:
+
+``` sh
+make latexpdf
+```
+
+- html:
+
+``` sh
+make latexpdf
+```
+
+The output of these build commands goes into subdirectory `build`.
+
+Status
+======
+
+The first release of the specification has been published. We are currently in
+the implementation phase, looking at various target projects, including U-Boot,
+coreboot, TF-A and Tianocore. Once this is done we will review the result to see
+if any serious flaws have come to light, meaning that changes are needed.  If
+so, these will be undertaken in each project.
+
+By the end of 2023, the spec will be considered stable and will be published
+as version 1. From there on, backwards compatibility will be maintained.