Add instructions on adding new Actions workflows

[damccorm]

Once this is merged, I'll send an email to the dev list with similar instructions

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Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI or the workflows README to see a list of phrases to trigger workflows.

[damccorm]

R: @Abacn @andreydevyatkin @tvalentyn @liferoad

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[tvalentyn]

Thanks a lot!