Readme: Add Presto-Cal link

Author: helgibbons

README.md

@@ -50,3 +50,4 @@ Links to community projects and other resources that you might find helpful can
 - [Presto attitude indicator for X-Plane12](https://github.com/PaulskPt/Presto_attitude_indicator_for_X-Plane12)
 - [Presto as an MQTT subscriber device](https://forums.pimoroni.com/t/presto-as-mqtt-subscriber-device/28051)
 - [Presto Flip Clock](https://forums.pimoroni.com/t/presto-flip-clock/28089)
+- [Presto-Cal - timezone/DST library with iCalendar parsing](https://forums.pimoroni.com/t/micropython-timezone-and-dst-library/28318)