Update our documentation for the new compression methods introduced in Kombu 4.3

[thedrow]

Description

The new release of Kombu 4.3 introduced a few new optional compression methods.
They are currently not documented in Celery.

Suggestions

We should document those new compression methods and instruct how to enable them.

[huangsam]

kombu.compression reveals the compression options available in Kombu v4.3.0 at the moment. Where would these settings be documented? It seems like the new options would need to be documented in the README.rst and introduction.rst of the Celery repo.

[thedrow]

https://github.com/celery/celery/blob/master/docs/userguide/calling.rst#compression - this is the right place.

[huangsam]

@thedrow I went ahead and created a PR to add compression scheme documentation at the location you pointed out.