Microsoft is probably already working on this as a default feature....but for now here is a chrome extension to add markdown to Microsoft Planner's Note section. This is still in development so pull requests welcome.

To learn how to use markdown here is a cheatsheet: Markdown Cheatsheet

For Devs: Clone the repo to your desktop and then npm install will install the respective dependencies. Use NPM Run Watch or NPM Run Build and then select the project folder and add to chrome as an unpacked extension.