clarification for README.md

points out that 'invalid' in this context refers to an invalid
_value_ for a _known_ option - as opposed to an _unknown option_.

PR-URL: #39

README.md

@@ -138,8 +138,8 @@ config object and remove its invalid properties.
 
 ## Error Handling
 
-By default, nopt outputs a warning to standard error when invalid
-options are found.  You can change this behavior by assigning a method
+By default, nopt outputs a warning to standard error when invalid values for
+known options are found.  You can change this behavior by assigning a method
 to `nopt.invalidHandler`.  This method will be called with
 the offending `nopt.invalidHandler(key, val, types)`.