minor #15722 [Config] move feature description to changelog file (xabbuh)

This PR was merged into the 2.8 branch.

Discussion
----------

[Config] move feature description to changelog file

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #15433
| License       | MIT
| Doc PR        |

Commits
-------

e1818bd move feature description to changelog file

UPGRADE-2.8.md

@@ -399,23 +399,3 @@ FrameworkBundle
        session:
            cookie_httponly: false
    ```
-
-Config
-------
-
-The edge case of defining just one value for nodes of type Enum is now allowed:
-
-```php
-$rootNode
-    ->children()
-        ->enumNode('variable')
-            ->values(array('value'))
-        ->end()
-    ->end()
-;
-```
-
-Before: `InvalidArgumentException` (variable must contain at least two
-distinct elements).
-After: the code will work as expected and it will restrict the values of the
-`variable` option to just `value`.

src/Symfony/Component/Config/CHANGELOG.md

@@ -1,6 +1,26 @@
 CHANGELOG
 =========
 
+2.8.0
+-----
+
+The edge case of defining just one value for nodes of type Enum is now allowed:
+
+```php
+$rootNode
+    ->children()
+        ->enumNode('variable')
+            ->values(array('value'))
+        ->end()
+    ->end()
+;
+```
+
+Before: `InvalidArgumentException` (variable must contain at least two
+distinct elements).
+After: the code will work as expected and it will restrict the values of the
+`variable` option to just `value`.
+
 2.7.0
 -----