Well, thanks to the holy folks at xda, we already have a couple of ROMs based on the just-released Android 6.0 Marshmallow MRA58K build for Nexus 5.
Android 6.0 Marshmallow factory images are out, but we’re pretty sure many of you folks are either waiting for OTA update or a recovery flashable Marshmallow ROM for your Nexus 5.
The author of this site or the developers are not responsible, if you damage or brick your device. Do it on your own risk and follow the instruction properly.
How to Install Android 6.0 Marshmallow ROM on Nexus 5.
1- Download and transfer both the Marshmallow ROM file and the new Bootloader + Radio zip file to your Nexus 5.
2- Boot into TWRP recovery.
3- Wipe data/Factory reset and Wipe Cache + Dalvik Cache.
4- First flash the MRA58K_Radio+Bootloader.zip file and then the hammerhead_mra58k.zip ROM file.
5- Reboot phone.
Congratulation You are now on Android 6.0 Marshmallow Rom