Easiest way is to disable systemd-resolved.service, and then set the DNS servers in network manager directly. See @pebcak 's answer.
Edit:
Personally, I set up my DNS servers manually using systemd-resolved in order to take advantage of caching.
To set your DNS servers in systemd-resolved, you simply have to add this file /etc/systemd/resolved.conf.d/dns.conf with the following contents:
[Resolve]
DNS="put your DNS ip here"
Fallback="put your fallback DNS ip here"
Domains=~.