Google My Business now lets you add a description to your business listing

Google Finally allows Businesses to add their business description to their knowledge panel

Google finally announces that businesses can now

To use this feature, log in to your Google My business account and click on the info button on the navigation bar. You will see a new field called Add Business Description.

Click on the little pencil edit box next to it and a box will popup allowing you to add your description of your business. 

It looks like they limited it to 750 characters, but that should do for most businesses.