[path_provider] Updated documentation reflecting changes needed for testing (#2815)

The recent federation of the Linux path provider might break tests, this updates the README.md to address those changes and direct the users how to mock the plugin in their tests.

Author: TimWhiting

packages/path_provider/path_provider/CHANGELOG.md

@@ -1,3 +1,6 @@
+## 1.6.11
+* Updated documentation to reflect the need for changes in testing for federated plugins
+
 ## 1.6.10
 * Linux implementation endorsement
 

packages/path_provider/path_provider/README.md

@@ -2,7 +2,8 @@
 
 [![pub package](https://img.shields.io/pub/v/path_provider.svg)](https://pub.dartlang.org/packages/path_provider)
 
-A Flutter plugin for finding commonly used locations on the filesystem. Supports iOS and Android.
+A Flutter plugin for finding commonly used locations on the filesystem. Supports iOS, Android, Linux and MacOS.
+Not all methods are supported on all platforms.
 
 ## Usage
 
@@ -19,3 +20,17 @@ String appDocPath = appDocDir.path;
 ```
 
 Please see the example app of this plugin for a full example.
+
+### Usage in tests
+
+`path_provider` now uses a `PlatformInterface`, meaning that not all platforms share the a single `PlatformChannel`-based implementation. 
+With that change, tests should be updated to mock `PathProviderPlatform` rather than `PlatformChannel`.
+
+See this `path_provider` [test](https://github.com/flutter/plugins/blob/master/packages/path_provider/path_provider/test/path_provider_test.dart) for an example.
+
+You will also have to temporarily add the following line to the setup of your test.
+```dart
+disablePathProviderPlatformOverride = true;
+```
+
+See this [issue](https://github.com/flutter/flutter/issues/52267), for more details on why this is needed.

packages/path_provider/path_provider/pubspec.yaml

@@ -2,7 +2,7 @@ name: path_provider
 description: Flutter plugin for getting commonly used locations on the Android &
   iOS file systems, such as the temp and app data directories.
 homepage: https://github.com/flutter/plugins/tree/master/packages/path_provider/path_provider
-version: 1.6.10
+version: 1.6.11
 
 flutter:
   plugin: