If you use macOS, it's also in the menu bar in the View menu.

This means that you can define an app-specific keyboard shortcut in Preferences > Keyboards > Shortcuts > App Shortcuts. Then click [+] and select Chrome as the application, then type "Enter Reader Mode" (exactly like it appears in the menu, and without quotes) and choose a shortcut.

I'd also recommend adding one for "Exit Reader Mode" since the option changes to this text once you are in reader mode. You can use the same shortcut, it's fine since the two options are never present at the same time.

This works for pretty much all apps, by the way.