Mouse and keyboard control for Xbox One users is still in development according to Xbox head Phil Spencer. After a couple months of silence on the issue, Spencer responded to a random tweet giving an update on the situation...

I would imagine it's not too far off as Microsoft is already implementing cross play with Windows 10 users on some games. If they were smart they would prep up a release of Age of Empires to boot with it! I'd love to finally play a true RTS on a console!