Add README link to documentation page (#9)

Author: Mitchman215

README.md

@@ -3,3 +3,5 @@ To build this example package:
     git clone https://github.com/apple/example-package-dealer.git
     cd example-package-dealer
     swift run
+
+For more information, visit [Swift's package manager documentation](https://www.swift.org/package-manager/).