Add comment explaining disk resizing
Add comment explaining disk resizing

This was cobbled together from various sources including:
  - https://stackoverflow.com/a/66117279/6995854
  - https://medium.com/@kanrangsan/how-to-automatically-resize-virtual-box-disk-with-vagrant-9f0f48aa46b3
  - https://marcbrandner.com/blog/increasing-disk-space-of-a-linux-based-vagrant-box-on-provisioning/

with some trial and error to get the minimum set of instructions.
We've done something similar in https://github.com/azavea/cicero/pull/1617

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