MINOR Add notes on upgrading for Convert::json2array() changes

docs/en/changelogs/3.0.0.md

@@ -113,6 +113,38 @@ As with any SilverStripe upgrade, we recommend database backups before calling `
 See [mysql.com](http://dev.mysql.com/doc/refman/5.5/en/converting-tables-to-innodb.html) for details on the conversion.
 Note: MySQL has made InnoDB the default engine in its [5.5 release](http://dev.mysql.com/doc/refman/5.5/en/innodb-storage-engine.html).
 
+### Convert::json2array() changes ###
+
+Convert JSON functions have been changed to use built-in json PHP functions `json_decode()` and `json_encode()`
+
+Because `json_decode()` will convert nested JSON structures to arrays as well, this has changed the way it worked,
+as before nested structures would be converted to an object instead.
+
+So, given the following JSON input to `Convert::json2array()`:
+
+	{"Joe":"Bloggs","Tom":"Jones","My":{"Complicated":"Structure"}}
+
+Here's the output from SilverStripe 2.4, with nested JSON as objects:
+
+	array(
+		'Joe' => 'Bloggs'
+		'Tom' => 'Jones',
+		'My' => stdObject(
+			Complicated => 'Structure' // property on object
+		)
+	)
+
+Now in SilverStripe 3.x, nested structures are arrays:
+
+	array(
+		'Joe' => 'Bloggs',
+		'Tom' => 'Jones',
+		'My' => array(
+			'Complicated' => 'Structure' // key value on nested array
+		)
+	)
+
+
 ### GridField: Replacement for TableListField and ComplexTableField ###
 
 We have a new component for managing lists of objects: The `[GridField](/topics/grid-field)`.