README: Point at Swift tools 5.1 branch

Should avoid folks with issues like mine: Peter-Schorn#21.

README.md

@@ -30,7 +30,7 @@ Read the full [documentation][1] and check out [this example iOS app][14] and th
 
 ## Supported Platforms
 
-* Swift 5.3+
+* Swift 5.3+ (for Swift 5.1 support, use the [`swift-tools-5-1` branch](https://github.com/Peter-Schorn/SpotifyAPI/tree/swift-tools-5-1))
 * iOS 13+
 * macOS 10.15+
 * tvOS 13+
@@ -302,4 +302,4 @@ This authorization process is implemented in this [example command-line app][23]
 
 [22]: https://github.com/Peter-Schorn/SpotifyAPIExampleApp#how-the-authorization-process-works
 [23]: https://github.com/Peter-Schorn/SpotifyAPIExamples
-[24]: https://developer.spotify.com/documentation/web-api/reference/
+[24]: https://developer.spotify.com/documentation/web-api/reference/