Add documentation for plugin treshhold handling

Fixes #15

Author: LordHepipud

doc/10-Icinga-Plugins.md

@@ -1,6 +1,18 @@
 # Icinga Plugins
 
-Below you will find a documentation for every single available plugin provided by this repository
+Below you will find a documentation for every single available plugin provided by this repository. Most of the plugins allow the usage of default Icinga threshold range handling, which is defined as follows:
+
+| Argument | Throws error on | Ok range                     |
+| ---      | ---             | ---                          |
+| 20       | < 0 or > 20     | 0 .. 20                      |
+| 20:      | < 20            | between 20 .. ∞              |
+| ~:20     | > 20            | between -∞ .. 20             |
+| 30:40    | < 30 or > 40    | between {30 .. 40}           |
+| `@30:40  | ≥ 30 and ≤ 40   | outside -∞ .. 29 and 41 .. ∞ |
+
+Please ensure that you will escape the `@` if you are configuring it on the Icinga side. To do so, you will simply have to write an *\`* before the `@` symbol: \``@`
+
+To test thresholds with different input values, you can use the Framework Cmdlet `Get-IcingaHelpThresholds`.
 
 * [Invoke-IcingaCheckBiosSerial](plugins/01-Invoke-IcingaCheckBiosSerial.md)
 * [Invoke-IcingaCheckCheckSum](plugins/02-Invoke-IcingaCheckCheckSum.md)