Update README to reflect dev branch

Author: wtdcode

README.md

@@ -52,5 +52,7 @@ If you want to contribute, please pick up something from our [Github issues](htt
 
 We also maintain a list of more challenged problems in [milestones](https://github.com/unicorn-engine/unicorn/milestones) for our regular release.
 
+Please send pull request to our [dev branch](https://github.com/unicorn-engine/unicorn/tree/dev).
+
 [CREDITS.TXT](CREDITS.TXT) records important contributors of our project.