Add minimal README.

third_party/.gitignore

@@ -5,6 +5,7 @@
 # Include the .gitignore file itself and .clang-tidy.
 !.gitignore
 !.clang-tidy
+!README.md
 
 # Include folders that have hand-written code (not DEPS).
 !accessibility/

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}/
+```