Added 'Contribute Code' section.

Added instructions oh how to contribute to the project (ie make pull requests to 'testing' branch instead of directly to 'master') so that if people want to contribute code, they can do it the right way. This makes community development much more easier and also prevents future code conflicts while merging this branch to 'master'.

Author: PrinceShaji

README.md

@@ -30,6 +30,7 @@ I disabled the issue section because of the flood of invalid questions. All the
 - [FAQ](#faq)
 - [License](#license)
 - [Sources and additional links](#sources-and-additional-links)
+- [Contribute Code](#contribute-code)
 
 ## Introduction ##
 
@@ -292,3 +293,7 @@ packet injection with ESP8266:
 802.11w-2009: https://en.wikipedia.org/wiki/IEEE_802.11w-2009
 
 Wi-Fi_send_pkt_freedom function limitations: https://esp32.com/viewtopic.php?t=586
+
+## Contribute Code
+
+To contribute code, make pull requests in `testing` branch to avoid future code conflicts. The `testing` branch is merged with `master` on every release.