Add upgrading section to reference docs with a link to the wiki

[philwebb]

Users might not find the release notes if they start with the reference docs. As suggested by @Buzzardo we should add a link.

[philwebb]

We should also add a link from the project README

[mpens]

Hi @philwebb i would like to help, do you think this information could be in the "How to" Guide section as a new item or we need to add complete new section?

[philwebb]

Thanks for the offer @mpens, but since this is on an older branch and there's quite a few hoops to jump through when merging PRs I've gone ahead and made the edits directly.