Update (August 23rd, 2016)
Since posting this little guide, lot’s of Pi’s have popped up doing Roon Bridge duties – great!
Two major HAT manufacturers have released certified Roon Ready images for their HATs (also great ;-)). These images just need to be written to an SD card and are ready for use. Both are currently in their 4th iteration and – as far as I can tell after running both for a few weeks – do what it says on the tin.
Hifiberry
You can download the image here. This is a closed image – no login possible or necessary). WiFi can be configured by side loading wpa_supplicant.conf on the boot partition of the SD card (this is a DOS partition that can be written to from any OS). More info on the Hifiberry community forums here.
IQ Audio
IQ Audio offers their image in the downloads section of their website (currently IQ_Roon_RC4.zip). IQ Audio’s image is accessible with the usual passwords (pi/raspberry). WiFi can be configured in the usual way, outlined in the guide above.
Alternative guide
Community member Jan Müller (@nikipore) has written a nice how-to for setting up a Pi w/ Hifiberry DAC+, with more elaborate Linux finetuning (Alsa configuration for Hifiberry and USB DACs, writing logs to RAM in order to prevent card corruption, making copies of your SD card by cloning) and instructions for installing Shairport-sync for Airplay support in in parallel with Roon Bridge. More information can be found here.