readme: add cron usage example

README.md

@@ -3,6 +3,15 @@
 Adjust maximum CPU frequency based on environment temperature.
 Works with [temphumd](https://git.ch1p.io/homekit.git/tree/src/temphumd.py) server.
 
+## Usage
+
+The script is supposed to be run by cron.
+
+Crontab line example, running it every 20 minutes:
+```
+*/20 * * * * /usr/local/bin/armbian-envtemp-cpufreq.sh -c /etc/armbian-envtemp-cpufreq.conf >/dev/null
+```
+
 ## Config example
 
 ```