Add documentation on the release process

Signed-off-by: Kate Goldenring <kate.goldenring@fermyon.com>

release.md

@@ -0,0 +1,50 @@
+# Cutting a new release of the Containerd Shim Spin
+
+To cut a new release of the `containerd-shim-spin`, you will need to do the
+following:
+
+1. Confirm that [CI is
+   green](https://github.com/spinkube/containerd-shim-spin/actions) for the
+   commit selected to be tagged and released.
+
+1. Change all references of the version number in package
+   [Cargo.toml](./Cargo.toml), crate
+   [Cargo.toml](./containerd-shim-spin/Cargo.toml),
+   [quickstart](./containerd-shim-spin/quickstart.md), [README](./README.md),
+   [deployments](./deployments/), and [images](./images/). Run `cargo build
+   --release` to make sure lockfiles reflect Cargo.toml updates. Add a new
+   column to the [README shim and Spin version
+   map](./README.md#shim-and-spin-version-map) that lists the version of the
+   Spin dependencies for the release.
+
+1. Create a pull request with these changes and merge once approved.
+
+1. Checkout the commit with the version bump from above.
+
+1. Create and push a new tag with a `v` and then the version number.
+
+    As an example, via the `git` CLI:
+
+    ```
+    # Create a GPG-signed and annotated tag
+    git tag -s -m "Containerd Shim Spin v0.15.0" v0.15.0
+
+    # Push the tag to the remote corresponding to spinkube/containerd-shim-spin (here 'origin')
+    git push origin v0.15.0
+    ```
+
+1. Pushing the tag upstream will trigger the [release
+   action](https://github.com/spinkube/containerd-shim-spin/actions/workflows/release.yaml).
+    - The release build will create binary releases of the shim and upload these
+      assets to a new GitHub release for the pushed tag. Release notes are
+      auto-generated but edit as needed especially around breaking changes or
+      other notable items.
+    - The release action also creates test applications, a k3d node image with
+      the `containerd-shim-spin`, and a new node installer image to be used by
+      the runtime class manager.
+  
+1. Update [SpinKube documentation](https://github.com/spinkube/documentation) as
+   necessary. Ensure the latest [node installer
+   image](https://www.spinkube.dev/docs/spin-operator/installation/installing-with-helm/#prepare-the-cluster)
+   is used and update the [Shim and Spin version
+   map](https://www.spinkube.dev/docs/containerd-shim-spin/).