docs: Add missing link to release process
Add a reference to the release.md file to make it more discoverable.
Fixes:
# Release tools
* [Introduction](#introduction)
+* [Create a Kata Containers release](#create-a-kata-containers-release)
* [`update-repository-version.sh`](#update-repository-versionsh)
* [Update Kata projects to a new version](#update-kata-projects-to-a-new-version)
* [`tag_repos.sh`](#tag_repossh)
This directory contains tools for Kata Containers releases.
+## Create a Kata Containers release
+
+See [the release documentation](release.md).
+
## `update-repository-version.sh`
This script creates a GitHub pull request (a.k.a PR) to change the version in
