docs: github publisher does not support update.electronjs.org

[kilbot]

update.electronjs.org requires latest-mac.yml files to work. The github piublisher does not produce latest.yml files so update.electronjs.org will not work.

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

The documentation about the GitHub Publisher is misleading, you can not use update.electronjs.org unless you create your own latest-mac.yml files.

[kilbot]

I see now that the problem is more that electron-forge and electron-updater are incompatible. Which is guess makes sense as electron-updater seems to be tightly coupled to electron-builder.

Perhaps there should be a 'gotchas' section for people migrating from electron-updater, because the auto-update process seems like taking a step backwards.