Update readme with useful resources

Author: AvdLee

README.md

@@ -4,4 +4,7 @@ A changelog generator written in Swift for GitHub repositories.
 
 - `cd` into the repository
 - run `swift package generate-xcodeproj` (Generates an Xcode project for development)
-- run `swift run` to test out any changes
+- run `swift run` to test out any changes
+
+### Useful resources
+- [Building a command line tool using the Swift Package Manager](https://www.swiftbysundell.com/articles/building-a-command-line-tool-using-the-swift-package-manager/)