Add alternative installation instructions to README

Provide an option to manually copy script files for installation. This helps users who prefer not to use the repository URL method for adding the package.

README.md

@@ -15,6 +15,10 @@ This package adds definition for the Google Stadia Controller to Unity's Input S
 `https://github.com/sim2kid/UnityStadiaController.git?path=Assets/Scripts/Package/StadiaInput`
 To set a target version, use the release tag like `#1.0.0` at the end of the repo string.
 
+### Alternative Installation
+If desired, you can instead install this package by copying the script files into your own repository.
+All script files are at `Assets/Scripts/Package/StadiaInput`.
+
 ## Required Packages
 * Unity Input System >= 1.11.2