[macOS] Add README.md for macOS embedder (flutter#40032)

Describes how to build and test the macOS embedder.

ci/licenses_golden/excluded_files

@@ -250,6 +250,7 @@
 ../../../flutter/shell/platform/darwin/Doxyfile
 ../../../flutter/shell/platform/darwin/common/framework/Source/flutter_codecs_unittest.mm
 ../../../flutter/shell/platform/darwin/common/framework/Source/flutter_standard_codec_unittest.mm
+../../../flutter/shell/platform/darwin/macos/README.md
 ../../../flutter/shell/platform/darwin/macos/framework/Source/fixtures
 ../../../flutter/shell/platform/embedder/fixtures
 ../../../flutter/shell/platform/embedder/platform_view_embedder_unittests.cc

shell/platform/darwin/macos/README.md

@@ -0,0 +1,58 @@
+# Flutter macOS Embedder
+
+This directory contains files specific to the Flutter macOS embedder. The
+embedder is built as a macOS framework that is linked into the target
+application. The framework is composed both of macOS-specific code and code
+shared with the iOS embedder. These can be found in:
+```
+flutter/shell/platform/darwin/common/framework
+flutter/shell/platform/darwin/macos/framework
+```
+
+Additionally, the framework relies on utility code shared across all embedders,
+which is found in:
+```
+flutter/shell/platform/common
+```
+
+To learn more, see the [Engine architecture wiki][wiki_arch].
+
+## Building
+
+Building all artifacts required for macOS occurs in two steps:
+* Host binary build: Builds tooling used by the Flutter tool to build Flutter
+  applications targeting macOS.
+* macOS Target build: Builds the framework that implements the macOS Flutter
+  embedder and exposes public API such as `FlutterViewController` used by
+  Flutter applications.
+
+Once you've [prepared your environment for engine development][wiki_engine_env],
+you can build the macOS embedder from the `src/flutter` directory using the
+following commands:
+```sh
+# Perform the host build.
+./tools/gn --unopt --no-goma
+autoninja -C ../out/host_debug_unopt      
+
+# Perform the macOS target build.
+./tools/gn --unopt --mac --no-goma
+autoninja -C ../out/mac_debug_unopt
+```
+Builds are architecture-specific, and can be controlled by specifying
+`--mac-cpu=arm64` or `--mac-cpu=x64` (default) when invoking `gn`.
+
+Googlers can remove `--no-goma` to make use of the Goma distributed compile
+service.
+
+## Testing
+
+The macOS-specific embedder tests are built as the
+`flutter_desktop_darwin_unittests` binary. Like all gtest-based test binaries, a
+subset of tests can be run by applying a filter such as
+`--gtest_filter='FlutterViewControllerTest.*Key*'`.
+
+More general details on testing can be found on the [Wiki][wiki_engine_testing].
+
+[wiki_arch]: https://github.com/flutter/flutter/wiki/The-Engine-architecture
+[wiki_engine_env]: https://github.com/flutter/flutter/wiki/Setting-up-the-Engine-development-environment
+[wiki_engine_testing]: https://github.com/flutter/flutter/wiki/Testing-the-engine