added contribution guide (#514)

* added contributing guide with info on setting up environment

* updated readme in root of repo to link to contribution guide

CONTRIBUTING.md

@@ -0,0 +1,17 @@
+## Environment setup
+
+`flutter_appauth` uses [Melos](https://melos.invertase.dev) to manage the monorepo project.
+
+To install Melos, run the following command from a terminal/command prompt:
+
+```
+dart pub global activate melos
+```
+
+At the root of your locally cloned repository bootstrap the all dependencies and link them locally
+
+```
+melos bootstrap
+```
+
+This removes the need for providing manual [`dependency_overrides`](https://dart.dev/tools/pub/pubspec). There's no need to run `flutter pub get` either. All the packages, example app and tests will run for the locally cloned repository. The workflows setup on GitHub are also configured use Melos to validate changes. For more information on Melos, refer to its [website](https://melos.invertase.dev)

README.md

@@ -2,4 +2,8 @@
 A Flutter plugin that provides a wrapper for native AppAuth SDKs (https://appauth.io) used authenticating and authorizing users. The repository consists of the following folders
 
 - [flutter_appauth](https://github.com/MaikuB/flutter_appauth/tree/master/flutter_appauth): code for the plugin
-- [flutter_appauth_platform_interface](https://github.com/MaikuB/flutter_appauth/tree/master/flutter_appauth_platform_interface): the code for common platform interface
+- [flutter_appauth_platform_interface](https://github.com/MaikuB/flutter_appauth/tree/master/flutter_appauth_platform_interface): the code for common platform interface
+
+## Contributing
+
+If you are interested in contributing, please review the [contribution guide](https://github.com/MaikuB/flutter_appauth/blob/master/CONTRIBUTING.md)