Document the prune command

[carolynvs]

Once #819 is merged, let's add documentation for subpackages to the README and explain how dep treats them. It's a bit confusing right now (before the ensure overhaul) that the lock lists the subpackages but they aren't automatically removed by dep. You must run dep prune to get rid of unused subpackages from vendor.

[sdboyer]

Yes, prune needs its own docs. Though they should also include a note - on both the CLI and whatever we write elsewhere - that we plan to subsume prune into ensure.

[stephenafamo]

Can I work on this? @carolynvs

[carolynvs]

@stephenafamo Yes, please that would be very helpful! ❤️

[sdboyer]

i'm gonna close this one out, as we're on the verge of getting rid of prune anyway.