Add example for merge! to README.md

Author: ASnow

README.md

@@ -67,6 +67,19 @@ end
 # => {"author": { "name": "David" }}
 ```
 
+
+To merge existing hash or array to current context:
+
+``` ruby
+hash = { author: { name: "David" } }
+json.post do
+  json.title "Merge HOWTO"
+  json.merge! hash
+end
+
+# => "post": { "title": "Merge HOWTO", "author": { "name": "David" } }
+```
+
 Top level arrays can be handled directly.  Useful for index and other collection actions.
 
 ``` ruby