ensure user is on the main channel

Author: chrisirhc

README.md

@@ -237,6 +237,8 @@ that's a good prompt to do this.  We also do this when there's a
 new PR merged that we particularly want to take.
 
 To update the version bounds:
+* First, make sure you're on the `main` channel, run:
+  `flutter channel main`.
 * Use `flutter upgrade` to upgrade your local Flutter and Dart.
 * Update the lower bounds at `environment` in `pubspec.yaml`
   to the new versions, as seen in `flutter --version`.