CPLAT-4109: Update documentation for dart2

CONTRIBUTING.md

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+# Contributing
+
+> We are working on a more thorough version of these contributing guidlines. For
+> the time being, we've documented our temporary dual-release setup that is
+> necessary for supporting Dart 1 and Dart 2 concurrently.
+
+## Dart 1 & Dart 2 Dual-Releases
+
+Long story short, we are unable to maintain a single codebase of over_react that
+is compatible with both Dart 1 and Dart 2. Transformers only work on Dart 1 and
+builders only work on Dart 2, and we can't maintain the transformer and builder
+together due to dependency conflicts.
+
+> For more information on our Dart 2 migration, [see this guide](/doc/dart2_migration.md).
+
+Fortunately, this problem can be shouldered entirely by the maintainers via a
+dual-release strategy (credit to the [dart2_constant](https://pub.dartlang.org/packages/dart2_constant)
+for the idea). The result is that consumers will depend on a version range of
+this library like they normally do and everything will _just work_.
+
+For every _version_ we want to release, we will actually release two releases -
+a Dart 1 (transformer) version and a Dart 2 (builder) version. These two
+releases will have the same semantic version, but will have unique build
+suffixes. The `pub` client will then decide which one to install based on their
+`environment.sdk` constraints and the active Dart SDK version.
+
+### Branches
+
+- `master`
+  - Dart2-only
+  - Provides a builder
+  - Environment constraint: `>=2.1.0 <3.0.0`
+
+- `master_dart1`
+  - Dart1-only
+  - Provides a transformer
+  - Environment constraint: `>=1.24.3 <2.0.0`
+
+### Release Process
+
+> Note that these steps are intended to be followed by Workiva employees, and as
+> such have some references to internal tooling.
+
+For every release, do the following:
+
+1. Ensure the next release versions exist in MARV:
+
+    Name | Branch
+    ---- | ------
+    over_react 2.x.x-dart1 | master_dart1
+    over_react 2.x.x-dart2 | master
+
+1. Trigger the `over_react 2.x.x-dart1` release first and review the PR:
+
+   - Ensure the updated `pubspec.yaml` version is correct, including the
+     `-dart1` suffix.
+
+   - Ensure the build passes.
+
+1. Trigger the `over_react 2.x.x-dart2` release second and review the PR:
+
+   - Ensure the updated `pubspec.yaml` version is correct, including the
+     `-dart2` suffix.
+
+   - Ensure the build passes.
+
+   - **Add any necessary changelog updates to the Dart 2 version.**
+
+1. **Merge the Dart 1 release first and publish it to pub.**
+
+1. Merge the Dart 2 release second and publish it to pub.
+
+1. Re-recreate the Dart 1 release in MARV (it does not get recreated
+   automatically like the default release does).
+
+