Getting OTA update once phone is rooted
You can also just reinstall the official recovery image by downloading the same factory image as currently running and extracting the recovery.img that's contained within.

Then you could reinstall the official recovery image by booting into fastboot mode and doing an:

    fastboot flash recovery recovery.img


This should give you back a working OTA mechanism. However note that you need to re-root after the update because this will undo the rooting procedure.