There were a few posts on the Syncthing forums requesting it as a feature but they didn't really get anywhere:
https://forum.syncthing.net/t/possibility-to-sync-local-fold...
In the mean time, I'm using GoodSync for sync'ing locally:
http://www.goodsync.com/download
It works quite well for that purpose, but since it's proprietary I'm not comfortable with blindly trusting it to sync my important files over the internet.
Still waiting for that perfect open-source p2p sync app that can also do local sync'ing.