Add a contributing guide (#1115)

Author: brianquinlan

CONTRIBUTING.md

@@ -0,0 +1,47 @@
+# Contributing :heart:
+
+Want to contribute? Great! First, read this page (including the small print at
+the end).
+
+### Before you contribute
+
+Before we can use your code, you must sign the
+[Google Individual Contributor License Agreement](https://cla.developers.google.com/about/google-individual)
+(CLA), which you can do online. The CLA is necessary mainly because you own the
+copyright to your changes, even after your contribution becomes part of our
+codebase, so we need your permission to use and distribute your code. We also
+need to be sure of various other things—for instance that you'll tell us if you
+know that your code infringes on other people's patents. You don't have to sign
+the CLA until after you've submitted your code for review and a member has
+approved it, but you must do it before we can put your code into our codebase.
+
+Before you start working on a larger contribution, you should get in touch with
+us first through the issue tracker with your idea so that we can help out and
+possibly guide you. Coordinating up front makes it much easier to avoid
+frustration later on.
+
+### Code reviews
+
+All submissions, including submissions by project members, require review.
+
+### File headers
+
+All files in the project must start with the following header.
+
+```dart
+// Copyright (c) 2024, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+```
+
+### The small print
+
+Contributions made by corporations are covered by a different agreement than the
+one above, the
+[Software Grant and Corporate Contributor License Agreement](https://developers.google.com/open-source/cla/corporate).
+
+## A word about conduct
+
+We pledge to maintain an open and welcoming environment :hugs:.
+For details, see our
+[code of conduct](https://dart.dev/community/code-of-conduct).

README.md

@@ -15,3 +15,8 @@ and the browser.
 | [cronet_http](pkgs/cronet_http/) | An Android Flutter plugin that provides access to the [Cronet](https://developer.android.com/guide/topics/connectivity/cronet/reference/org/chromium/net/package-summary) HTTP client. | [![pub package](https://img.shields.io/pub/v/cronet_http.svg)](https://pub.dev/packages/cronet_http) |
 | [cupertino_http](pkgs/cupertino_http/) | A macOS/iOS Flutter plugin that provides access to the [Foundation URL Loading System](https://developer.apple.com/documentation/foundation/url_loading_system). | [![pub package](https://img.shields.io/pub/v/cupertino_http.svg)](https://pub.dev/packages/cupertino_http) |
 | [flutter_http_example](pkgs/flutter_http_example/) | An Flutter app that demonstrates how to configure and use `package:http`. | — |
+
+## Contributing
+
+If you'd like to contribute to any of these packages, see the
+[Contributing Guide](CONTRIBUTING.md).