add "keepalive-workflow"

[tyler36]

"
GitHub will suspend the scheduled trigger for GitHub action workflows if there is no commit in the repository for the past 60 days. The cron based triggers won't run unless a new commit is made. It shows the message "This scheduled workflow is disabled because there hasn't been activity in this repository for at least 60 days" under the cronjob triggered action.
"

DDEV addons will become stable at some point and no longer require frequent commits. However, we still need to runs tests against newer versions of DDEV.

Enter https://github.com/gautamkrishnar/keepalive-workflow

We need to:

• add keepalive workflow to workflow template
• document why it is there

[rfay]

It's a great idea to get it in here first.

[tyler36]

We can sort out issues here first.
ddev-addon-template will probably need to use it too at some point.

[rfay]

Fixed in #5