If you're a Windows Insider, starting with yesterday's Build 14371 you can link your Windows 10 license with your Microsoft account (MSA). The feature is already activated if you use your MSA to login to Windows 10. If you don't, simply do so and you should be on your way.
The change solves two issues: needing to keep your license key stored somewhere and being unable to activate Windows if you've lost your key. It also pushes the operating system even further into the digital realm for the better.
The bad news is licenses are still limited (how much exactly we don't know yet), but it's an understandable necessity given how easy it would be to share a license infinitely with your login details.