Document the use of apispec[marshmallow]

[Jérôme Lafréchoux]

See #706 (comment).

[Sinclert Pérez]

(Continuing the conversation here)

Thanks for the issue tracking @lafrech 😄

I would say both solutions work. Either:

• The installation of apispec[marshmallow] is specified on the README and this page of the documentation.
• The dependency marshmallow is moved to the install_requires section of the setup.py.

I, personally, am a bigger fan of the second one, given that.... most? examples provided in the docs involve marshmallow. However, you are the maintainer, so it is your choice.

[Jérôme Lafréchoux]

I opened #708 to discuss second option.

Docs still need improvement for now.