Update README.md with release time process doc

[andrew-m-leonard]

Add a verbose section on how to process dockerhub images at release time, and a general basic explanation of how it works for newbies...

Fixes #382

[github-actions]

A block has been put on this Pull Request as this repository is temporarily under a code freeze due to an ongoing release cycle.

If this pull request needs to be merged during the release cycle then please comment /merge and a PMC member will be able to remove the block.

If the code freeze is over you can remove this block by commenting /thaw.

[sxa]

I'm not personally going to approve this but others can if they so desire. This is adding an extra section with similar instructions about how the process works and the new section still requires you to look at the other section to know how to do the required bits so you have to bounce around if you look at the new instructions and this introduces a certain amount of duplication which risks things getting out of sync.

My preference would be to update the existing section in this document if it's unclear with additional points for clarity and/or put a terse version of this in the releasing guide.

[andrew-m-leonard]

/merge

[github-actions]

Approval to merge during the lockdown cycle

Please can two Adoptium PMC members comment /approve?

Code freeze removed

[sxa]

My previous review still stands. Figuring out the process now involves bouncing around the document with separate sections instead of trying to make the existing sections clearer, but I won't block it.