Add instructions for recreating virtualenv
Add instructions for recreating virtualenv

Since this only affects users who run `git pull`, and applies for both
kits, I am adding it to the repo's README.md. In the long-run, if we
switch to create a PyPI package, pip will take care of dependencies.

This addresses the following issues:
https://github.com/google/aiyprojects-raspbian/issues/209
https://github.com/google/aiyprojects-raspbian/issues/249

Change-Id: I11ded46d9630f860bebf91e060845d0acf4c42df

google/aiyprojects-raspbian,google/aiyprojects-raspbian,google/aiyprojects-raspbian,google/aiyprojects-raspbian,google/aiyprojects-raspbian