Merge pull request #3335 from divayprakash/fix-yaml

[yaml/en] Explain regexp, closes #2197

yaml.html.markdown

@@ -121,6 +121,10 @@ other_anchor: *anchor_name
 base: &base
   name: Everyone has same name
 
+# The regexp << is called Merge Key Language-Independent Type. It is is used to
+# indicate that all the keys of one or more specified maps should be inserted
+# into the current map.
+
 foo: &foo
   <<: *base
   age: 10