Update release docs to include process for patch releases

The doc is a simplified version of the one used for Tekton Pipelines,
without the use of milestones and specially labelled issues. This is
something we may consider in future but are not currently using.

tekton/README.md

@@ -10,6 +10,7 @@ Tekton! This directory contains the
 that we use.
 
 * [How to create a release](#create-an-official-release)
+* [How to create a patch release](#create-a-patch-release)
 * [Automated nightly releases](#nightly-releases)
 * [Setup releases](#setup)
 * [npm packages](#npm-packages)
@@ -18,6 +19,21 @@ that we use.
 
 To create an official release, follow the steps in the [release-cheat-sheet](./release-cheat-sheet.md)
 
+## Create a patch release
+
+Sometimes we'll find bugs that we want to backport fixes for into previous releases
+or discover things that were missing from a release that are required by upstream
+consumers of a project. In that case we'll make a patch release. To make one:
+
+1. Create a branch for the release named `release-<version number>.x`, e.g. [`release-v0.18.x`](https://github.com/tektoncd/dashboard/tree/release-v0.18.x)
+   and push it to the repo https://github.com/tektoncd/dashboard (you may need help from
+   [an OWNER](../OWNERS) with permission to push) if that release branch does not exist.
+1. Use [git cherry-pick](https://git-scm.com/docs/git-cherry-pick) to cherry pick the
+   fixes from main into the release branch you have created (use `-x` to include
+   the original commit information).
+1. [Create an official release](#create-an-official-release) for the patch, with the
+   [patch version incremented](https://semver.org/)
+
 ## Nightly releases
 
 The nightly release pipeline is