Update source install instructions

[andrewmkiss]

Recently was installing pyxrf in a clean py39 venv. I followed the instructions on the readthedocs, but still needed to manually add some dependencies.

Expected Behavior

I followed the instructions posted here http://nsls-ii.github.io/PyXRF/installation.html
I started with a new py39 venv. Cloned the repo, installed PyQt, and then pip installed pyxrf. I expected pyxrf to run after that, but I ran into dependency issues

Current Behavior

I needed to manually install some dependencies (scikit-beam and xraylib) in order to get pyxrf to run.

Possible Solution

Can the requirements.txt file be updated to include these dependencies?