Update RG description in release checklist

[michaelgrund]

Description of proposed changes

Expand description of RG announcement regarding uploading the released version as zip-file.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash command is:

• /format: automatically format and lint the code

[seisman]

also please see #3692.

I think changes here should be merged into that PR.

[seisman]

I'm merging this PR into #3692 and marking PR #3692 as ready for review.