[docs] More details documentation with best practices

[netrix]

Could you prepare more detailed documentation of how tox-pdm should actually be embedded into a python repository?

This is a great project and it makes working with pdm much easier but it lacks proper documentation of how to actually place it in a project.

In general pdm add -d tox-pdm will add it to the project configuration but that means it will be checked each time tox is called because it's a dependency itself and to actually use it a person need to run pdm install before calling tox.

On the other hand installing it globally means that its version must be somehow in sync with pdm. It looks like both packages are independent if looking at configuration but I noticed that in my project both version diverged and actually tox was not able to install packages that were not available in common PyPI but from private server.

Since tox-pdm actually requires a pdm to work I think it is worth adding it as a dependency that will actually lock to a version that was tested.