Several articles and blog posts have been written about using the NMRA DCC Decoder library and you can access them directly from this page.
17 Function DCC Decoder
Geoff Bunza wrote a series of articles about building some DCC Decoders on Geoff Bunza’s Scale Model Animation Blog and here are the direct links:
SMA 13 – Update to the 17 Pin Configurable Multi Function Decoder / Accessory Decoder Version Added
SMA12 – 17 Channel Configurable Multifunction $5 DCC Decoder For Servos
SMA10 – Build a 17-Function DCC Decoder for about $5
17 Function configurable decoder: servos, stepper, LEDs, & sound
A detailed construction article also appeared in the on-line magazine:
Model Railroader Hobbyist, March 2017, p.204, DCC projects using the Arduino
Mark Riddoch has written an article Servos, DCC & Arduinos. It first appeared in the MERG Journal – June 2015 Vol 49 No. 2 and is also made available here with Mark’s permission.
Petr Šídlo has posted an article describing a DCC Accessory Decoder that can drive up to 16 relays on his website here. It’s written in Czech language but Google Translate does a pretty good job…