Start documenting for non-anaconda users

[JoeGermuska]

I wanted to run these tools, but I am not an Anaconda user.

I adapted requirements.txt to the pip format, creating the new pip-requirements.txt. By way of crude documentation, I moved the non-python dependencies to the bottom of this new file, commented out. I didn't actually need to install any of the non-python dependencies, but, then, over time, I may have already installed many of them. So, for a "cleaner" system, there may still be things people need to install. Of course, documenting that part of the process will be much more complicated than it is for Anaconda users.

I tried to explain it all in a new section of README.md.

I tested it by running the three demo commands in README.md and got no errors, although I didn't pay much attention to what was actually retrieved.