Merge in plugin README and CONTRIBUTING (#3252)

* Merge in plugin contribution docs

I forgot to merge this content in when merging the repositories.

* Minor changes

* Update README as well

CONTRIBUTING.md

@@ -1,9 +1,3 @@
-# Contributing to Flutter Packages
-
-[![Build Status](https://api.cirrus-ci.com/github/flutter/packages.svg)](https://cirrus-ci.com/github/flutter/packages/main)
-
-_See also: [Flutter's code of conduct](https://github.com/flutter/flutter/blob/master/CODE_OF_CONDUCT.md)_
-
 ## Welcome
 
 For an introduction to contributing to Flutter, see [our contributor
@@ -16,15 +10,32 @@ Additional resources specific to the packages repository:
 - [Packages repository structure](https://github.com/flutter/flutter/wiki/Plugins-and-Packages-repository-structure),
   to get an overview of how this repository is laid out.
 - [Contributing to Plugins and Packages](https://github.com/flutter/flutter/wiki/Contributing-to-Plugins-and-Packages),
-  for more information about how to make PRs for this repository.
+  for more information about how to make PRs for this repository, especially when
+  changing federated plugins.
+- [Plugin tests](https://github.com/flutter/flutter/wiki/Plugin-Tests), which explains
+  the different kinds of tests used for plugins, where to find them, and how to run them.
+  As explained in the Flutter guide,
+  [**PRs need tests**](https://github.com/flutter/flutter/wiki/Tree-hygiene#tests), so
+  this is critical to read before submitting a plugin PR.
 
 ## Notes
 
 ### Style
 
-Flutter packages follow [Flutter's style
-guide](https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo), with the
-exception that code is formatted with `dart format`.
+Flutter packages and plugins follow Google style—or Flutter style for Dart—for the languages they
+use, and use auto-formatters:
+- [Dart](https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo) formatted
+  with `dart format`
+- [C++](https://google.github.io/styleguide/cppguide.html) formatted with `clang-format`
+  - **Note**: The Linux plugins generally follow idiomatic GObject-based C
+    style. See [the engine style
+    notes](https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style)
+    for more details, and exceptions.
+- [Java](https://google.github.io/styleguide/javaguide.html) formatted with
+  `google-java-format`
+- [Objective-C](https://google.github.io/styleguide/objcguide.html) formatted with
+  `clang-format`
+- [Swift](https://google.github.io/swift/) formatted with `swift-format`
 
 ### Releasing