Document how to deal with GitHub disabling CompatHelper workflows after 60 days of inactivity

[JeffFessler]

GitHub disables actions after 60 days of inactivity and there is discourse about how to address it but I cannot quite tell what it the appropriate solution:
https://discourse.julialang.org/t/compathelper-workflows-disabled-after-60-days-of-inactivity/48870

It would be great if the README here would address this issue specifically and definitively because an Oct. 2020 comment there suggests that a CompatHelper action will be "effectively deprecate[d]" but I still see lots of actively maintained packages using it so I doubt that prediction has come true yet
https://discourse.julialang.org/t/compathelper-workflows-disabled-after-60-days-of-inactivity/48870/9

Should we package developers be doing anything other than using the latest CompatHelper.yml file?

[mattBrzezinski]

Just commented on this.

We've created an AWS Lambda solution for this problem, KeepActionsAlive, which automatically re-enables your workflows.