[video_player] Document how to use pigeon and update to the latest version

packages/video_player/video_player/CONTRIBUTING.md

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+## Updating pigeon-generated files
+
+If you update files in the pigeons/ directory, run the following
+command in this directory:
+
+```bash
+flutter pub get
+flutter pub run pigeon --dart_null_safety --input pigeons/messages.dart
+(cd ../../../; ./script/incremental_build.sh format --travis --clang-format=clang-format-7)
+```
+
+If you update pigeon itself, temporarily update the pubspec.yaml by
+adding the following to the `dependency_overrides` section, assuming
+you have checked out the `flutter/packages` repo in a sibling
+directory to the `plugins` repo:
+
+```bash
+  pigeon:
+    path:
+      ../../../../packages/packages/pigeon/
+```
+
+Then, run the commands above. When you run `pub get` it should warn
+you that you're using an override. If you do this, you will need to
+publish pigeon before you can land the updates to this package, since
+the CI tests run the analysis using latest published version of
+pigeon, not your version or the version on master.
+
+In either case, the configuration will be obtained automatically from
+the `pigeons/messages.dart` file (see `configurePigeon` at the bottom
+of that file).
+
+While contributing, you may also want to set the following dependency
+overrides:
+
+```yaml
+dependency_overrides:
+  video_player_platform_interface:
+    path:
+      ../video_player_platform_interface
+  video_player_web:
+    path:
+      ../video_player_web
+```
+
+## Publishing plugin updates that span multiple plugin packages
+
+If your change affects both the interface package and the
+implementation packages, then you will need to publish a version of
+the plugin in between landing the interface changes and the
+implementation changes, since the implementations depend on the
+interface via pub.
+
+To do this, follow these steps:
+
+1. Create a PR that has all the changes, and update the
+`pubspec.yaml`s to have path-based dependency overrides as described
+in the "Updating pigeon-generated files" section above.
+
+2. Upload that PR and get it reviewed and into a state where the only
+test failures relate to the cross-package dependencies. (There'll be a
+bunch of failures of this nature.)
+
+3. Create a PR that's a subset of the one in the previous step that
+only includes the interface changes, with no dependency overrides, and
+submit that.
+
+4. Once you have had that reviewed and landed, publish the interface
+parts of the plugin to pub.
+
+5. Now, update the original full PR to not use dependency overrides
+but to instead refer to the new version of the plugin, and sync it to
+master (so that the interface changes are gone from the PR). Submit
+that PR.
+
+6. Once you have had _that_ PR reviewed and landed, publish the
+implementation parts of the plugin to pub.