Document the use of `mamba` for solving environments

[Michele Peresano]

I see that this already popped out in issues like #240, but I felt the need to ask/suggest explicitly.

Ae already using mamba to create the conda envs? If not I'd suggest to do it as it is sensibly faster.
You could use the mambaforge distribution.

[Dan Foreman-Mackey]

Since v0.4.2 (this PR, specifically: #268) mamba is supported as follows:

showyourwork build --conda-frontend=mamba

but that's not really documented anywhere so it would be great to add that!