update README

Author: chrisirhc

README.md

@@ -92,26 +92,21 @@ Two specific points to expand on:
 
 ### Setting up
 
-1. Follow the [Flutter installation guide](https://docs.flutter.dev/get-started/install)
-   for your platform of choice.
-2. Switch to the latest version of Flutter by running `flutter channel main`
-   and `flutter upgrade` (see [Flutter version](#flutter-version) below).
+1. Install direnv (TBA), and set up its hook.
+2. Make sure you have initialized submodules by doing (do
+not do `--depth=1` as flutter relies on tags to know its version):
+
+  ```sh
+  git submodule update --init
+  ```
 3. Ensure Flutter is correctly configured by running `flutter doctor`.
 4. Start the app with `flutter run`, or from your IDE.
 
 
 ### Flutter version
 
-While in the beta phase, we use the latest Flutter from Flutter's
-main branch.  Use `flutter channel main` and `flutter upgrade`.
-
-We don't pin a specific version, because Flutter itself doesn't offer
-a way to do so.  So far that hasn't been a problem.  When it becomes one,
-we'll figure it out; there are several tools for this in the Flutter
-community.  See [issue #15][].
-
-[issue #15]: https://github.com/zulip/zulip-flutter/issues/15
-
+We pin to a particular version of flutter SDK via git submodules in [vendor/flutter/](vendor/flutter/).
+If your local checkout of this repository does not have this submodule checked out at the same commit, the build may fail.
 
 ### Tests
 
@@ -248,7 +243,8 @@ To update the version bounds:
 * Run `flutter pub get`, which will update `pubspec.lock`.
 * Make a quick check that things work: `tools/check`,
   and do a quick smoke-test of the app.
-* Commit and push the changes in `pubspec.yaml` and `pubspec.lock`.
+* Commit and push the changes in the submodule `vendor/flutter`,
+  `pubspec.yaml` and `pubspec.lock`.
 
 
 ### Upgrading dependencies