virtual environment section of the README.md

[paciorek]

@kaneplusplus a couple questions:

1. There is an incomplete sentence "Virtual environments may be". Can you fix this?
2. I'm wary of suggesting that users give us submit a container image file given how large it might be.
3. I'm not sure of what "shared folder" means.

[paciorek]

More generally, let's discuss the recommendations for package env'ts and virtual env'ts at the Friday meeting. I think it's worth some discussion to make sure we're all on the same page.

[paciorek]

Ok, I edited this in light of our discussion.

We probably want to have some further discussion at some point to think more carefully about what recommendations to make about virtual environments and containers.