Document how & when to manually publish to RubyGems

This came up today when trying to release a patch to v3 of
govuk_app_config, which currently has v4 in pre-release. The
process to manually work around the Jenkins gem publishing should
be documented.

source/manual/publishing-a-ruby-gem.html.md

@@ -47,6 +47,32 @@ The default Jenkins build script will automatically detect the presence of a
 [Jenkinsfile for govuk_app_config](https://github.com/alphagov/govuk_app_config/blob/master/Jenkinsfile)
 for an example.
 
+### Manually publishing gems from the CLI
+
+Sometimes you may be required to publish a gem outside of Jenkins, if you need
+to release a patch or minor version below the current latest major version.
+For example, say you have a gem at version 1.0.0 and later release a major breaking
+change as version 2.0.0. If you still need to support apps using v1, and you want
+to apply a bugfix as a patch version 1.0.1, you would not be able to deploy off
+the `main` branch as this will include all of the breaking v2 changes.
+
+To manually publish from the command line:
+
+1. Get your branch ready: checkout the tag of the previous major version (e.g.
+   `git checkout v1.0.0`) and then create a branch pointing at this commit
+   (`git checkout -b bump-version-to-1.0.1`).
+1. Get your code ready: make your code changes, including adding a CHANGELOG entry
+   and bumping the version number.
+1. Commit.
+1. Manually follow the steps of the [`publishGem` function in govuk-jenkinslib](https://github.com/alphagov/govuk-jenkinslib/blob/c25c362dd8288e92a09b6f6cc9b5dd6fd6c12c84/vars/govuk.groovy#L762-L806)
+   i.e. check your new version doesn't already exist, build & publish the gem,
+   make and push a git tag. The RubyGems credentials can be found in govuk-secrets.
+1. Tidy up: open a pull request for this branch against `main`. There will be merge
+   conflicts, which you'll have to manually resolve before you can merge. Do this
+   **without rebasing**, as we want to keep the original `main` history. You'll end
+   up with a meta commit like `Merge pull request #1 from username/branch-name`,
+   but the full history of both `main` and your branch will be intact.
+
 ## Clearing the gemstash cache
 
 When a new gem version is released, it may not be available immediately on Jenkins.