Add simple support for tags in YAML I/O

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194644 91177308-0d34-0410-b5e6-96231b3b80d8

docs/YamlIO.rst

@@ -633,6 +633,20 @@ This works for both reading and writing. For example:
     };
 
 
+Tags
+----
+
+The YAML syntax supports tags as a way to specify the type of a node before
+it is parsed. This allows dynamic types of nodes.  But the YAML I/O model uses
+static typing, so there are limits to how you can use tags with the YAML I/O
+model. Recently, we added support to YAML I/O for checking/setting the optional 
+tag on a map. Using this functionality it is even possbile to support differnt 
+mappings, as long as they are convertable.  
+
+To check a tag, inside your mapping() method you can use io.mapTag() to specify
+what the tag should be.  This will also add that tag when writing yaml.
+
+
 Sequence
 ========