Document that onChangeInput is only called when inputValue is set

It's a bit confusing that this function isn't called if you don't set inputValue, it's probably good to be explicit about that in the readme. We found out the solution by looking at this issue olahol#123

Author: Hatlen

README.md

@@ -232,6 +232,7 @@ tag array, `changed` which is an array of the tags that have changed and
 ##### onChangeInput
 
 Callback from the input box, gets one argument `value` which is the content of the input box.
+(onChangeInput is only called if the input box is [controlled](https://facebook.github.io/react/docs/forms.html#controlled-components), for this to happen both inputValue and onChangeInput need to be set)
 
 ##### addKeys
 
@@ -243,7 +244,7 @@ A string to set a value on the input.
 
 ##### inputValue
 
-Similar to `currentValue` but needed for controlling the input box.
+Similar to `currentValue` but needed for controlling the input box. (inputValue is only useful if you use it together with onChangeInput)
 
 ##### onlyUnique