Support table on forward/backward compatibility of PyGMT with Python and GMT versions

[weiji14]

Description of the desired feature

To be nice to our users and clear up any potential source of confusion on PyGMT's compatibility with GMT, we should have a table or something indicating forward/backward compatibility. It's a bit hard to pick thing out from the changelog at https://www.pygmt.org/latest/changes.html. Something like so, inspired by https://github.com/holoviz/pyviz_comms#compatibility (correct me if I'm wrong):

PyGMT	GMT	Notes
0.1.0	>= 6.0.0, < 6.1.0?	#363, requires GMT modern mode
0.1.1	>= 6.0.0, < 6.1.0?
0.2.0	>= 6.1.1	#577, new earth relief pixel registration default
0.2.1	>= 6.1.1
0.3.0	>= 6.1.1 ?
0.4.0	>= 6.2.0 ?

See thread at #729 (comment)_ for context. Probably should add a column for Python compatibility too, related to issue #690.

Are you willing to help implement and maintain this feature? Discuss first on where to put this, maybe at https://github.com/GenericMappingTools/pygmt#documentation-for-other-versions?

[seisman]

Sounds a great idea, although it makes the README even longer 😄