Mention `pipenv run pew toggleglobalsitepackages` in the docs

I’m currently working on a project where pipenv is a great fit for managing local testing environments, but some of the relevant Python bindings (dnf, rpm, libsolv) aren’t currently pip-installable.

While it makes sense to me that #419 was closed as being out-of-scope, I think it may be worth explicitly mentioning @vorstelok’s workaround from https://github.com/kennethreitz/pipenv/issues/419#issuecomment-331013455 for folks in situations like mine, as it makes it clear that the strongly preferred answer is to make the affected components pip installable, but there’s still a defined way to weaken the isolation from the system level components when you’re actually working on system level tools.

However, there’s a slight simplification available: rather than use pew workon, it’s possible to just use pipenv run or pipenv shell as you would with any other command.

[kennethreitz]

We support it in venv too!

[ncoghlan]

Even better, thanks!

I guess having this be an existing feature in pew does change the trade-offs in also offering it as a feature in pipenv 😃