Once you've downloaded the plugin, head to Chrome > Preferences > Extensions to make sure it's enabled. Then go forth and freely read each and every message, undetected by the sender. Just remember this only works on the desktop version of the site (for now, you're on your own when it comes to that stupid Messenger app).
You may be thinking, won't it seem suspicious if I reply to someone's message without a read receipt showing up first? Yeah, probably, but not to worry -- the plugin has you covered. There's an option that allows you to manually mark individual messages as read, on demand.