Fix git-flow hyperlink style

README.md

@@ -113,10 +113,12 @@ Actual constants are type-safe, have more explicit scope (they’re not availabl
 
 ### Branching Model
 
-Especially when distributing an app to the public (e.g. through the App Store), it's a good idea to isolate releases to their own branch with proper tags. Also, feature work that involves a lot of commits should be done on its own branch. [`git-flow`](https://github.com/nvie/gitflow) is a tool that helps you follow these conventions. It is simply a convenience wrapper around Git's branching and tagging commands, but can help maintain a proper branching structure especially for teams. Do all development on feature branches (or on `develop` for smaller work), tag releases with the app version, and commit to master only via
+Especially when distributing an app to the public (e.g. through the App Store), it's a good idea to isolate releases to their own branch with proper tags. Also, feature work that involves a lot of commits should be done on its own branch. [`git-flow`][gitflow-github] is a tool that helps you follow these conventions. It is simply a convenience wrapper around Git's branching and tagging commands, but can help maintain a proper branching structure especially for teams. Do all development on feature branches (or on `develop` for smaller work), tag releases with the app version, and commit to master only via
 
     git flow release finish <version>
 
+[gitflow-github]: https://github.com/nvie/gitflow
+
 ## Common Libraries
 
 Generally speaking, make it a conscious decision to add an external dependency to your project. Sure, this one neat library solves your problem now, but maybe later gets stuck in maintenance limbo, with the next OS version that breaks everything being just around the corner. Another scenario is that a feature only achievable with external libraries suddenly becomes part of the official APIs. In a well-designed codebase, switching out the implementation is a small effort that pays off quickly. Always consider solving the problem using Apple's extensive (and mostly excellent) frameworks first!