README: Add section on how to update from the official mirror

[hnez]

The following needs to happen before this PR can be merged:

• [ ] Deploy a test bundle on downloads.linux-automation.com
• [ ] Possibly update the dummy URL in this PR
• [ ] Test the update command mentioned here

[hnez]

I've cherry-picked and updated the changes from this PR in #38, which already had related changes.