Add missing checklist step to Release Checklist template

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Add missing checklist step to Release Checklist template:

Task `Navigate and edit the GitHub release. Attach the composed source maps to the release (you can drag and drop the files in the “Attach binaries” drop area). Once they are uploaded, update the artifact’s name following this format:

• https://github.com/wordpress-mobile/release-toolkit-gutenberg-mobile/issues/165