Add note about expectation that Shorebird be installed for fastlane use (#323)

Author: bryanoltman

src/content/docs/ci/fastlane.mdx

@@ -17,6 +17,10 @@ Follow the setup instructions on the fastlane website to install fastlane
 
 ## Setup
 
+This guide assumes that you have shorebird installed on the machine that will be
+running the fastlane commands. If you have not already installed shorebird, you
+can do so by following the [Getting Started instructions](/#install).
+
 If you are not already using fastlane with your project, navigate to your app's
 `ios` directory in your project and run `fastlane init`. You will be prompted to
 answer several questions. For this guide, we will choose to manually add lanes