Update README.md

added appointment.one to schedulers

Author: jorn-k

README.md

@@ -67,6 +67,7 @@ Community-sourced tools for the DevRel industry. Brought to you by [The DevRel C
 * [x.ai](https://x.ai/): AI scheduled meetings for smarter work days. The future is here.
 * [vyte.in](https://www.vyte.in/): Schedule meetings 10x faster
 * [calendso](https://calendso.com/): Open source Calendly
+* [appointment.one](https://appointment.one/): Very good scheduler with multiple-calendar support and group scheduling options. Has a free tier.
 
 ## Virtual Events