Add notes re upgrade

Should probably merge some version of this now that 5.67 is out. Although I personally think the first paragraph should be removed since putting on a sysadmin hat I wouldn't know what it's telling me I need to do. Maybe a link to a lab ticket for background for those who like that.

Similarly the 3rd paragraph could say "From CiviCRM 5.67 onwards you may see blank text versions of some message templates since they will be generated automatically from the html version when the email is sent."

assigned to @eileen

mentioned in commit 55ce76d1

merged

Right - I agree that more has happened / it could be tweaked - but also that this is better in than out at this stage