docs: add the README file

README.md

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+# Chisel Releases <!-- omit from toc -->
+
+*The collection of package slice definitions, for
+[Chisel](https://github.com/canonical/chisel).*
+
+- [The Basics](#the-basics)
+- [Using a Specific Chisel Release](#using-a-specific-chisel-release)
+- [Adding New Slice Definitions](#adding-new-slice-definitions)
+
+## The Basics
+
+This repository is the official source of all package slice definitions that
+are supported by [Chisel](https://github.com/canonical/chisel).
+
+Every Chisel release is represented by a Git branch within this repository. At
+the moment, the officially supported Chisel releases are:
+
+- [ubuntu-20.04](https://github.com/canonical/chisel-releases/tree/ubuntu-20.04)
+\- Focal
+- [ubuntu-22.04](https://github.com/canonical/chisel-releases/tree/ubuntu-22.04)
+\- Jammy
+- [ubuntu-22.10](https://github.com/canonical/chisel-releases/tree/ubuntu-22.10)
+\- Kinetic (EOL)
+- [ubuntu-23.04](https://github.com/canonical/chisel-releases/tree/ubuntu-23.04)
+\- Lunar
+- [ubuntu-23.10](https://github.com/canonical/chisel-releases/tree/ubuntu-23.10)
+\- Mantic
+- [ubuntu-24.04](https://github.com/canonical/chisel-releases/tree/ubuntu-24.04)
+\- Noble
+
+In each release branch, you'll find a `chisel.yaml` file that defines the Chisel
+release, plus a `slices` folder with all the Slice Definitions Files (SDFs) for
+that release.
+
+For more information on the SDFs' YAML schema and how to install slices, please
+refer to the
+[Chisel documentation](https://github.com/canonical/chisel/blob/main/README.md).
+
+## Using a Specific Chisel Release
+
+Chisel releases are meant to be used with the `chisel` CLI. Many of the `chisel`
+commands have a `--release` optional argument (to know which commands support
+this option, please refer to the
+[Chisel documentation](https://github.com/canonical/chisel/blob/main/README.md)).
+
+When running a `chisel` command where `--release` is supported,
+
+- **if** `--release` is not given, Chisel will default to the host's Ubuntu
+release, mapping it to its corresponding branch in this repository. E.g.: if
+running the `chisel` command without `--release` on a Jammy host, Chisel will
+automatically default to the `ubuntu-22.04` Chisel release;
+- **if** `--release` is an absolute system path, then Chisel will look at the directory tree under that path to find a valid Chisel release (this is
+especially useful when you're working with custom Chisel releases and/or
+defining new slices);
+- **if** `--release` is a string that matches the `ubuntu-##.##` pattern, then
+Chisel will use the corresponding Git branch from this repository if it exists.
+
+## Adding New Slice Definitions
+
+We welcome and encourage community contributions! To better understand how to
+write and propose new package slice definitions, please read the
+[contributing guidelines](./CONTRIBUTING.md).