Do I need to flash a rooted kernel to root, or is it enough to flash a custom recovery? I want to root my Nexus 6 which is on Android M, would it be enough to flash TWRP and then SuperSU, or would I have to flash a specific kernel to root it? 
Also is TWRP 2.8.6.0 compatible with Android M?
For rooting your phone I found this nice guide (5/29/2015) with links to all necessary files. In the guide they are using TWRP 2.8.5, but since 2.8.6 is out it should work as well. If it doesn't work, just flash the version in the guide. SuperSU 2.46 is being used in the guide. Also remember that Android M (MPZ44Q) is only a developer preview, so it isn't a fully implemented release.

As far your kernel, NO you don't need a custom kernel. Only with a custom ROM, would that be necessary. 

Make sure your bootloader is unlocked 

fastboot oem unlock




glossary (urban-ish)


Root/rooting - gaining administrative control
Recovery - aids in administration, plus backups, sideloads , and etc.
Kernel - tells the hardware what to do, custom kernels allow changing variables
Boot image - preps the system for the ROM
ROM - the devices operating system




remember

1. Perform a backup

2. Unlocking boot loader will ERASE/WIPE/DELETE everything

3. YOU assume all responsibility in rooting your Nexus