[DOCS] Move the release checklist out of the README (#1391)

Part of #1360

README.md

@@ -492,21 +492,6 @@ more than welcome. :pray: Please have a look at our
 [contribution guidelines](CONTRIBUTING.md) to learn more about how to
 contribute to Emogrifier.
 
-## Steps to release a new version
-
-1. In the [composer.json](composer.json), update the `branch-alias` entry to
-   point to the release _after_ the upcoming release.
-1. In the [CHANGELOG.md](CHANGELOG.md), create a new section with subheadings
-   for changes _after_ the upcoming release, set the version number for the
-   upcoming release, and remove any empty sections.
-1. Update the target milestone in the Dependabot configuration.
-1. Create a pull request "Prepare release of version x.y.z" with those changes.
-1. Have the pull request reviewed and merged.
-1. Tag the new release.
-1. In the [Releases tab](https://github.com/MyIntervals/emogrifier/releases),
-   create a new release and copy the change log entries to the new release.
-1. Post about the new release on social media.
-
 ## Maintainers
 
 * [Oliver Klee](https://github.com/oliverklee)

docs/release-checklist.md

@@ -0,0 +1,14 @@
+# Steps to release a new version
+
+1. In the [composer.json](../composer.json), update the `branch-alias` entry to
+   point to the release _after_ the upcoming release.
+1. In the [CHANGELOG.md](../CHANGELOG.md), create a new section with subheadings
+   for changes _after_ the upcoming release, set the version number for the
+   upcoming release, and remove any empty sections.
+1. Update the target milestone in the Dependabot configuration.
+1. Create a pull request "Prepare release of version x.y.z" with those changes.
+1. Have the pull request reviewed and merged.
+1. Tag the new release.
+1. In the [Releases tab](https://github.com/MyIntervals/emogrifier/releases),
+   create a new release and copy the change log entries to the new release.
+1. Post about the new release on social media.