add docs: usage

Usage.md

+# Usage of amcli
+
+## Show help
+
+```text
+./src/amcli -h
+IML Appmonitor as CLI client 0.1
+
+This client performs appmonitor checks and puts the results as JSON to stdout.
+You can use the compiled binary on non PHP systems.
+(c) 2025 Institute for Medical education * University of Bern
+
+SYNTAX: amcli [OPTIONS] --ini=<INI-FILE>
+
+OPTIONS:
+    -h, --help      Print this help and exit
+    -i, --ini       Set an INI File to parse
+    -l, --list      list available checks and exit
+    -m, --modules   list available Php modules in this binary and exit
+    -v, --verbose   Enable verbose output
+    -V, --version   Show version and exit
+
+EXAMPLES:
+
+    amcli -i=my.ini
+    amcli --ini=my.ini
+        Execute checks from INI file 'my.ini'.
+
+    amcli --list
+        List available checks.    
+
+```
+
+## Execute checks
+
+`amcli --ini=<FILENAME>`
+
+## Exitcodes
+
+### amcli
+
+| Exitcode | Description
+| ---      | ---
+| 0        | OK. An option was executed (help, list modules or checks) or the check was executed successfully
+| 1        | include include_checks.php failed (php file only)
+| 2        | include_checks.php failed (php file only)
+| 3        | INI file was not given. Or missing `=` char in `--ini=<FILE>`
+| 4        | Given Ini file does not exist
+| 5        | Given Ini file cannot be parsed
+| 6        | Ini value `params=` is no JSON
+
+### appmonitor-client.class.php
+
+| Exitcode | Description
+| ---      | ---
+| 10       | incomplete reponse data detected eg. no check, no result code
+| 11       | Access denied: Ip restriction was set
+| 12       | Access denied: No token or wrong token was given
+
+### appmonitor-checks.class.php
+
+| Exitcode | Description
+| ---      | ---
+| 20       | Missing key in a given hash
+| 21       | Mising value in a given hash
+| 22       | Check not found. Check your `function`value
+| 23       | Check didn't response a hash (=error in check)
+| 24       | Check responded a too small hash (=error in check)
+| 25       | The unit in a space value is unknown
+