Microsoft has revised its policy concerning the development of apps in Windows 10. In statement made on the Windows Dev Center, Microsoft announced: "A developer license is no longer required for each device that you want to use to develop, install or test your app."
You are free to develop your custom app (using Visual Studio) or run non-Windows Store apps on your computer. However, in order to take advantage of the new developer features, you'll first need to enable Developer Mode in your Windows 10 settings.
Click on Start and go to Settings > Update & Security >For developers. Under Use developer features, simply select Developer Mode:
Click on Yes to confirm the activation of the Developer Mode:
N.B. You'll find more information pertaining to the development of Windows 10 apps on the Windows Dev Center.