I thought about the thing to have a rooted Nexus 5 with Android 5 but how??
The answer is: YES it is possible 😀
but Android 5 is for me not completely stable 😐 and it is stable 🙂
IF you have rooted your Nexus 5 with Andorid 5 your system will have an ram overflow after 5 to 15 minutes after power on!!!! – Wait with the update to Android 5 and the rooting of it.I have tried it a second time and after that it works for me now fine.
I have installed the Android 5 on my Nexus 5 with the how-to from the Google developer: Factory Images for Nexus on my Xubuntu-Notebook.