Updated README_cron.md

Author: hmaerki

cron/README_cron.md

@@ -13,13 +13,31 @@ Login as `githubrunner`
 crontab -e
 
 # m h dom mon dow   command
-0 20 * * * cd ~/testbed_micropython && ./cron/run_test_report_all.sh >> ~/testbed_micropython/cron/cron_log.txt 2>&1
+0 20 * * * cd /home/githubrunner/testbed_micropython && /home/githubrunner/testbed_micropython/cron/run_test_report_all.sh >> /home/githubrunner/testbed_micropython/cron/cron_log.txt 2>&1
 ```
 
 ## Debugging
 
+Crontab file
+
+```bash
+cat /var/spool/cron/crontabs/githubrunner
+
+crontag -l # list user's crontab
+```
+
+Test command
+
+
+```bash
+env -i /bin/bash --noprofile --norc
+<-- here goes to cron-command
+```
+
+Observer running jobs
+
 ```bash
 tail -f ~githubrunner/testbed_micropython/cron/cron_log.txt
 
 journalctl -t CRON | tail
-```
+```