[VarDumper] Add doc for assertDump* assertions

Author: nicolas-grekas

components/var_dumper/introduction.rst

@@ -73,6 +73,26 @@ current PHP SAPI:
     #. From time to time, run ``composer global update`` to have the latest
        bug fixes.
 
+Using VarDumper in your PHPUnit test suite
+------------------------------------------
+
+Since Symfony 2.7, the VarDumper component provides
+:class:`a trait <Symfony\\Component\\VarDumper\\Test\\VarDumperTestTrait>`
+that can help writting some of your tests for PHPUnit.
+By adding::
+
+    use \Symfony\Component\VarDumper\Test\VarDumperTestTrait;
+
+at the beginning of your test class declaration, you'll gain two new assertions:
+
+:method:`Symfony\\Component\\VarDumper\\Test\\VarDumperTestTrait::assertDumpEquals`
+    verifies that the dump of the variable given as second argument matches the
+    expected dump provided as a string in the first argument.
+
+:method:`Symfony\\Component\\VarDumper\\Test\\VarDumperTestTrait::assertDumpMatchesFormat`
+    is like the previous method but accepts placeholders in the expected dump,
+    based on the ``assertStringMatchesFormat`` method provided by PHPUnit.
+
 DebugBundle and Twig Integration
 --------------------------------