[zendframework#6154] Added note to README

(detailing BC break)

README.md

@@ -17,6 +17,13 @@ DD MMM YYY
 
 ### UPDATES IN 2.4.0
 
+- [#6154](https://github.com/zendframework/zf2/pull/6154) updates
+  `Zend\InputFilter\BaseInputFilter::isValid()` to accept an optional `$context`
+  parameter; if used, this value will be passed to all composed inputs as
+  context, instead of the value provided to `setData()`. For classes overriding
+  the `isValid()` method of an InputFilter, you will need to add
+  `$context = null` as an argument.
+
 Please see [CHANGELOG.md](CHANGELOG.md).
 
 ### SYSTEM REQUIREMENTS