Add a README with instructions for editing and running tests for the FGP (#161830)

Add a readme with some info.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

---------

Co-authored-by: Gray Mackall <mackall@google.com>
Co-authored-by: Reid Baker <hamilton.reid.baker@gmail.com>

packages/flutter_tools/gradle/README.md

@@ -0,0 +1,32 @@
+## The Flutter Gradle Plugin
+
+This directory contains Gradle code used to by the Flutter tool to build Flutter apps for Android,
+primarily the Flutter Gradle Plugin (FGP) - a Gradle plugin built on top of Gradle and the Android 
+Gradle Plugin (AGP).
+
+### Editing in Android Studio
+
+To get code completion in Android Studio, you must open a new Android Studio window with this 
+directory as the root. Code completion will not work when navigating to files 
+in an Android Studio window opened at the root of the entire Flutter repo.
+
+### Contributing
+
+The Flutter Gradle Plugin is [currently being re-written](https://github.com/flutter/flutter/issues/121541) from Groovy to Kotlin 
+(Kotlin source specifically, i.e. not `.kts`). As such, outside of critical bug fixes, 
+new contributions will only be accepted in `src/main/kotlin` (and `src/test/kotlin`).
+
+### Testing
+
+Tests can be run in Android Studio, or directly with Gradle: `./gradle test` 
+(note that this directory does not contain a version controlled Gradle file. You can find one in 
+the engines `third_party` directory at 
+`<flutter_root>/engine/src/flutter/third_party/gradle/bin/gradle`).
+
+Alternatively, you can run all the tests in one file by passing in the fully qualified class name, 
+e.g. `./gradlew test --tests com.flutter.gradle.BaseApplicationNameHandlerTest`, or one test in 
+one file by passing in the fully qualified class name followed by the method name, 
+e.g `./gradlew test --tests "com.flutter.gradle.BaseApplicationNameHandlerTest.setBaseName respects Flutter tool property"`.
+
+Sometimes changing a test name and then running it will cause an IDE error. To get Android Studio back
+to a good state on Mac, run `Help > "Repair IDE"`, and then in the popup window `"Rescan project indexes > Everything works now."`