Modified README.md with instructions on creating a profile file.

Found a bug in the AddOnMenu script. During tests the script worked fine, but in practice it was failing. Turns out I needed to use global variables as the scriptblocks when creating sub menus are like functions with their own scope of variables. Good learning experience, took about an hour of my time though before I stumbled upon the idea! Hadn't realized PowerShell too has these scopes.