Add a callout to the collection repr

[jbusecke]

Is your feature request related to a problem? Please describe.
I am currently restructuring the Pangeo/ESGF CMIP6 Cloud Data and would like for everyone to slowly migrate their workflows to this new catalog. Now I can change the documentation of the main entry points (docs/gh repos/...) but it is impossible to hunt down every instance of the 'legacy' catalog and replace it.

Describe the solution you'd like
I do however have access to that legacy json file, and am wondering if there is a way to add a 'message' item, which would then be displayed as a red callout when calling the collection html repr?
That way everyone who access the legacy catalog and runs a recent intake-esm version would get alerted that there is a new catalog whenever they rerun their code.

Would this be doable/resonable?

[mgrover1]

Thanks for opening the issue here - I like this idea!! Would you be interested in drafting a PR here?

[jbusecke]

Currently very limited in time, but could try to take a swing in late March!