CyanogenMod 12.1 nightlies rolling out to most devices
CM12.1 is based on Android 5.1 and it will install on top of CM 12. Any and all third-party add-on ZIPs will have to be updated manually if you find versions compatible to Android 5.1.
The CyanogenMod 12.1 build is already available for devices such as Moto G, OnePlus One, 2013 LTE Nexus 7, and Moto E. These nightlies will keep coming until most devices are cleared out.
A milestone CM build will show up soon, and that will make things more stable, if you wish to wait for one.