Add podspec

I’ve recently added [`RBStoryboardLink`](https://github.com/CocoaPods/Specs/commit/51a01fdf6fd000e6b9dbcd2b82cdbd4c2d220bf6) to the [CocoaPods](https://github.com/CocoaPods/CocoaPods) package manager repo.

CocoaPods is a tool for managing dependencies for OS X and iOS Xcode projects and provides a central repository for iOS/OS X libraries. This makes adding libraries to a project and updating them extremely easy and it will help users to resolve dependencies of the libraries they use.

However, `RBStoryboardLink` doesn't have any version tags. I’ve added the current HEAD as version 0.0.1, but a version tag will make dependency resolution much easier.

[Semantic version](http://semver.org) tags (instead of plain commit hashes/revisions) allow for [resolution of cross-dependencies](https://github.com/CocoaPods/Specs/wiki/Cross-dependencies-resolution-example).

This pull request adds the podspec to the repository, so that it can be versioned together with `RBStoryboardLink`.