Add patch release contribution docs to CONTRIBUTING.md. (#39357)

* Add patch release contribution docs to CONTRIBUTING.md.

Co-authored-by: Alex Arslan <[email protected]>
Co-authored-by: Viral B. Shah <[email protected]>

Author: 3 people

CONTRIBUTING.md

@@ -248,6 +248,51 @@ process before running the corresponding test. This can be useful as a shortcut
 on the command line (since tests aren't always designed to be run outside the
 runtest harness).
 
+### Contributing to patch releases
+
+The process of creating a patch release is roughly as follows:
+
+1. Create a new branch (e.g. `backports-release-1.6`) against the relevant minor release
+   branch (e.g. `release-1.6`). Usually a corresponding pull request is created as well.
+
+2. Add commits, nominally from `master` (hence "backports"), to that branch.
+   See below for more information on this process.
+
+3. Run the [BaseBenchmarks.jl](https://github.com/JuliaCI/BaseBenchmarks.jl) benchmark
+   suite and [PkgEval.jl](https://github.com/JuliaCI/PkgEval.jl) package ecosystem
+   exerciser against that branch. Nominally BaseBenchmarks.jl and PkgEval.jl are
+   invoked via [Nanosoldier.jl](https://github.com/JuliaCI/Nanosoldier.jl) from
+   the pull request associated with the backports branch. Fix any issues.
+
+4. Once all test and benchmark reports look good, merge the backports branch into
+   the corresponding release branch (e.g. merge `backports-release-1.6` into
+   `release-1.6`).
+
+5. Open a pull request that bumps the version of the relevant minor release to the
+   next patch version, e.g. as in [this pull request](https://github.com/JuliaLang/julia/pull/37718).
+
+6. Ping `@JuliaLang/releases` to tag the patch release and update the website.
+
+7. Open a pull request that bumps the version of the relevant minor release to the
+   next prerelase patch version, e.g. as in [this pull request](https://github.com/JuliaLang/julia/pull/37724).
+
+Step 2 above, i.e. backporting commits to the `backports-release-X.Y` branch, has largely
+been automated via [`Backporter`](https://github.com/KristofferC/Backporter): Backporter
+searches for merged pull requests with the relevant `backport-X.Y` tag, and attempts to
+cherry-pick the commits from those pull requests onto the `backports-release-X.Y` branch.
+Some commits apply successfully without intervention, others not so much. The latter
+commits require "manual" backporting, with which help is generally much appreciated.
+Backporter generates a report identifying those commits it managed to backport automatically
+and those that require manual backporting; this report is usually copied into the first
+post of the pull request associated with `backports-release-X.Y` and maintained as
+additional commits are automatically and/or manually backported.
+
+When contributing a manual backport, if you have the necessary permissions, please push the
+backport directly to the `backports-release-X.Y` branch. If you lack the relevant
+permissions, please open a pull request against the `backports-release-X.Y` branch with the
+manual backport. Once the manual backport is live on the `backports-release-X.Y` branch,
+please remove the `backport-X.Y` tag from the originating pull request for the commits.
+
 ### Code Formatting Guidelines
 
 #### General Formatting Guidelines for Julia code contributions