README: Add note on certificate installation for mac users

README.md

@@ -85,6 +85,8 @@ pip3 install -r requirements.txt
 
 To run the program on windows, open `run.bat`. On Linux/MacOS, run `python3 server.py`.
 
+**Note for Mac users**: If you installed Python from the installer on python.org, you will need to have run the file `Install Certificates.command` in the directory `Applications/Python 3.x` where `x` is the minor version of python. Otherwise, you will get the error `certificate verify failed: unable to get local issuer certificate`. There is a similar file in that directory you can run to get the `python3` command installed for the terminal.
+
 To run it at startup on Windows, right click `run.bat` and click "Create Shortcut." Then, move the shortcut to the Startup folder. You can access the Startup folder by pressing `Windows Key + R` and typing `shell:startup`.