Add requirements.txt listing external modules

[ewenmcneill]

To make it easier for more advanced Python users to know what needs to be installed, add a requirements.txt, and section to the README listing what is needed and why.

(The "auto install from running program" functionality is clever, but more advanced users may wish to create a venv or similar if they know external modules are needed and why.)

[ewenmcneill]

Thanks for writing this code and making it available so quickly. I'm very impressed with how well it's worked for "just in time" code.

Feel free to modify/merge/close this PR as you wish. I just figured a "can click merge" PR was more useful than an issue saying "you should..." :-)

Ewen

[timhutton]

It makes sense. I've added a comment to the front page about this, based on your suggestion. I prefer not to have a requirements file at this point.

Thanks!