Add docs for usage with variable dependencies in a matrix

[enpaul]

The below is a fairly typical setup used to test compatibility with multiple versions of a package:

[tox]
envlist = py3{7,8,9}-foo{1,2,3}

[testenv]
description = Some very cool tests
deps =
    foo1: foo == 1.0
    foo2: foo == 2.0
    foo3: foo == 3.0

This use case is supported and can exist alongside the plugin being installed. A section should be added to the Other Notes section explaining how.

(In the meantime see #58)