create markdown help for check_file_age

docs/20_Checks/check_file_age.md

+## Check filesystem errors
+
+Script: `check_file_age`
+
+**check_file_age** checks if files are not out of date.
+You can find non finisheing processes that do not write an output file or logfile anymore.
+
+This check sends performance data.
+
+## Requirements
+
+* sudo permission on /bin/journalctl to scan for the information
+
+```txt
+icingaclient ALL=(ALL) NOPASSWD: /bin/journalctl
+```
+
+## Standalone installation
+
+From this repository ypu need next to this script:
+
+* `inc_pluginfunctions` shared function for all IML checks written in bash
+
+## Syntax
+
+```txt
+______________________________________________________________________
+
+CHECK_FILE_AGE
+v1.0
+
+(c) Institute for Medical Education - University of Bern
+Licence: GNU GPL 3
+
+https://os-docs.iml.unibe.ch/icinga-checks/Checks/check_file_age.html
+______________________________________________________________________
+
+Check if files are not out of date.
+You can customize the values for
+* directory
+* limits for warning and critical
+* filename filter pattern
+
+This plugin sends performancedata.
+
+SYNTAX:
+
+  check_file_age [-h] [--dir PATH] [--filter FILTER] [--critical VALUE] [--warning VALUE]
+
+OPTIONS:
+
+  -h, --help 
+      this help
+
+PARAMETERS:
+
+  -d, --dir PATH
+      set installation dir of iml deployment to find its check skript
+      default dir: /var/iml-backup
+
+  -c, --critical VALUE
+      critical level in days (default: 14)
+
+  -w. --warning VALUE
+      warning level in days (default: 7)
+
+  -f --filter FILTER
+     filter for filenames (default: *.*
+
+EXAMPLE:
+
+  check_file_age
+      Check backup data with initial values
+
+  check_file_age -d /data/mybackups
+      Check iso files a given directory
+    
+  check_file_age -d /data/mybackups -w 14 -c 28
+      Check iso files a given directory and customized limits
+
+```
+
+### Parameters
+
+(none)