vpython: Add a list of relevant env vars to vpython's README.md.
vpython: Add a list of relevant env vars to vpython's README.md.

This seems like useful information for users, and I don't believe it's
documented in a single place. So this help resolve that.

And also insert a table-of-contents at the top of the doc.

Bug: None
Change-Id: If2f5c8e40f8a939ea9de05d6dfab1975af9460cf
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2885721
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Reviewed-by: Brian Ryner <388ea68edb4d8f934960c128120aee09a8f854bf@google.com>
Commit-Queue: Ben Pastene <e214258550f76fa23ac7c1d953424f3f468f5c6b@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go