Adding read me and commenting potential change to actions

[travjenkins]

Changes

• Adding a new label PR:Preview for when we want to actually run the preview step. That way we are not constantly running these jobs when we don't need to.

Tests / Screenshots

• Testing in Github with this PR.

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit a0db835):

https://estuary-marketing--pr230-travjenkins-actions-8221tpmf.web.app

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