This commit adds an explanation of the role of existing requirements.txt
files to the README.

I found this very unclear and opened #1358 because of my confusion. The
existing short note that requirements.txt "might interfere" didn't
really help me. I'm not sure how common my confusion is, but I hope my
changes makes things clearer to new users (assuming what I've written is
in fact correct!)

(In addition to my additions, I moved the "Updating requirements"
section up in the doc, since it seems like a core workflow.)