[#8200] document the !!str tag

Author: xabbuh

components/yaml/yaml_format.rst

@@ -308,6 +308,9 @@ The YAML specification defines some tags to set the type of any data explicitly:
 .. code-block:: yaml
 
     data:
+        # this value is parsed as a string (it's not transformed into a DateTime)
+        start_date: !!str 2002-12-14
+
         # this value is parsed as a float number (it will be 3.0 instead of 3)
         price: !!float 3
 
@@ -318,4 +321,7 @@ The YAML specification defines some tags to set the type of any data explicitly:
             Pz7Y6OjuDg4J+fn5OTk6enp
             56enmleECcgggoBADs=
 
+.. versionadded:: 3.4
+    Support for the ``!!str`` tag was introduced in Symfony 3.4.
+
 .. _YAML: http://yaml.org/