Google has officially released Android 5.1 Lollipop, which brings in several fixes and new features.
Android 5.1.0, (yes the zero in the name is real) adds support for Multiple SIM cards.
The latest iteration of Android, which was only available for Android One devices until now, adds Device Protection, which when enabled, will require you to sign in with your Google account, even after the phone had been reset to factory settings. This feature will be available for the Nexus 6, Nexus 9 and devices which shipped with 5.1
High Definition calls will be supported on devices like the Nexus 6 (Verizon or T-Mobile),and other compatible devices. The update also brings in some enhancements for Bluetooth and Wi-Fi, which you can sign in to from the quick settings.
Google has said that it has begun rolling out the update, so Nexus device owners could start receiving them anytime. If you do not wish to wait, you can flash a factory image of Android 5.1.0, which Google has posted on the official factory images page. Read more about it here.