Prepare to move new folders into third_party/ by adding a .gitignore.

ci/licenses_golden/excluded_files

@@ -391,6 +391,8 @@
 ../../../flutter/sky/tools/objcopy.py
 ../../../flutter/testing
 ../../../flutter/third_party/.clang-tidy
+../../../flutter/third_party/.gitignore
+../../../flutter/third_party/README.md
 ../../../flutter/third_party/accessibility/README.md
 ../../../flutter/third_party/accessibility/ax/ax_enum_util_unittest.cc
 ../../../flutter/third_party/accessibility/ax/ax_event_generator_unittest.cc

third_party/.gitignore

@@ -0,0 +1,20 @@
+# Ignore everything by default, as these come from gclient/DEPS.
+# We'll explicitly include the folders we want to track.
+*
+
+# Include the .gitignore file itself and .clang-tidy.
+!.gitignore
+!.clang-tidy
+!README.md
+
+# Include folders that have hand-written code (not DEPS).
+!accessibility/
+!canvaskit/
+!ninja/
+!spring_animation/
+!test_shaders/
+!tonic/
+!txt/
+!web_locale_keymap/
+!web_test_fonts/
+!web_unicode/

third_party/README.md

@@ -0,0 +1,24 @@
+# `flutter/third_party`
+
+This directory contains third-party code that is a combination of:
+
+- Code that is vendored into the Flutter repository, from an external source.
+  For example, we might have `third_party/glfw`, which contains the GLFW
+  library, vendored from an external repository.
+
+  > 💡 **TIP**: See [`DEPS`](../DEPS) for where these sources are declared.
+
+- Code that originates from another repository, but is copied (sometimes with
+  alterations) into the Flutter repository. For an example, see
+  [`third_party/spring_animation`](spring_animation/README.md).
+
+- Code that is licensed separately from the rest of the Flutter repository.
+  For example, see [`third_party/txt`](txt/).
+
+When adding a new _externally_ sourced third-party library, update `.gitignore`:
+
+```diff
+# Ignores all third_party/ directories except for the ones we want to track.
+
++ !{folder_name}/
+```