modified contributing.md to add git best practices link

CONTRIBUTING.md

@@ -9,6 +9,8 @@ Please go ahead and sign, putting your github username in "Address line #2", so
 
 ## Code Conventions
 
+To get started with developing for Signal iOS, please read [this tutorial on git best practices](https://gist.github.com/corbett/ef9fd5f1abbef3b02f3b).
+
 We are trying to follow the [GitHub code conventions for Objective-C](https://github.com/github/objective-c-conventions) and we appreciate that pull requests do conform with those conventions.
 
 In addition to that, always add curly braces to your `if` conditionals, even if there is no `else`. Booleans should be declared according to their Objective-C definition, and hence take `YES` or `NO` as values.