Added a note about the ARG_MUST_BE_DEFINED variable to the README

Author: moebrowne

README.md

@@ -43,6 +43,9 @@ argExpected['d|deamon|D']="argumentName - Argument description"
 
 The `argumentName` part of the definition is the name given to the argument and what should be passed to the `argValue` and `argExists` functions, see below.
 
+By default if an argument is passed that hasn't been defined an error will be thrown and the script will exit.
+This feature can be turned off by setting `ARG_MUST_BE_DEFINED` to `false`, note that the argument names will default to the argument its self, without the preceding hyphen(s).
+
 ### Get An Arguments Value
 
 There is a helper function named `argValue()` which takes the name of