[Form] Added missing entries to CHANGELOG and UPGRADE

CHANGELOG-2.1.md

@@ -165,6 +165,14 @@ To get the diff between two versions, go to https://github.com/symfony/symfony/c
  * allowed setting different options for RepeatedType fields (like the label)
  * added support for empty form name at root level, this enables rendering forms
    without form name prefix in field names
+   
+ * [BC BREAK] made form naming more restrictive. Form and field names must
+   start with a letter, digit or underscore and only contain letters, digits,
+   underscores, hyphens and colons
+
+ * [BC BREAK] changed default name of the prototype in the "collection" type
+   from "$$name$$" to "__name__". No dollars are appended/prepended to custom
+   names anymore.
 
  * [BC BREAK] greatly improved `ChoiceListInterface` and all of its
    implementations. `EntityChoiceList` was adapted, the methods `getEntities()`,

UPGRADE-2.1.md

@@ -122,3 +122,22 @@ UPGRADE FROM 2.0 to 2.1
                 {{ choice.label }}
             </option>
         {% endfor %}
+
+* In the template of the collection type, the default name of the prototype
+  field has changed from "$$name$$" to "__name__"
+
+    For custom names, no dollars are prepended/appended anymore. You are advised
+    to prepend and append double underscores wherever you have configured the
+    prototype name manually.
+
+    Before:
+
+        $builder->add('tags', 'collection', array('prototype' => 'proto'));
+
+        // results in the name "$$proto$$" in the template
+
+    After:
+
+        $builder->add('tags', 'collection', array('prototype' => '__proto__'));
+
+        // results in the name "__proto__" in the template