Add configuration instructions to readme

[natalie-robinson]

In the "How to use" section of the readme, the command to run is provided but there are no instructions for helping users configure their "knit" command (i.e., find the path to knit and set it in their environmental variables, zshrc, etc.). These instructions would be incredibly helpful.

Running on a Mac, I did the following (after installing TeXLive and pandoc and zen-knit):

1. From terminal, "mdfind knit"
2. Open .zshrc and add (based on the output of the above):
   "alias knit=/Users/myName/Library/Python/3.9/bin/knit"