Update README.md (#1700)

Added sign up link to App Center

Author: elamalani

README.md

@@ -5,6 +5,8 @@ We are prioritizing this work because we believe this to be the most valuable th
 
 [![appcenterbanner](https://user-images.githubusercontent.com/31293287/32969262-3cc5d48a-cb99-11e7-91bf-fa57c67a371c.png)](http://microsoft.github.io/code-push/)
 
+#### [Sign up With App Center](https://appcenter.ms/signup?utm_source=CodePush&utm_medium=Azure) to use CodePush
+
 # React Native Module for CodePush
 
 *Note: This README is only relevant to the latest version of our plugin. If you are using an older version, please switch to the relevant tag on [our GitHub repo](https://github.com/Microsoft/react-native-code-push) to view the docs for that particular version.*