README improvements (#250)

- The mention of testing in the README (which is what appears on PyPI)
  felt out of place. Replace it with a more general link to CONTRIBUTING.
- Mention why typing-extensions is special.
- Add discussion of stub files to CONTRIBUTING.

CONTRIBUTING.md

@@ -26,6 +26,14 @@ Starting with version 4.0.0, `typing_extensions` uses
 [Semantic Versioning](https://semver.org/). See the documentation
 for more detail.
 
+# Type stubs
+
+A stub file for `typing_extensions` is maintained
+[in typeshed](https://github.com/python/typeshed/blob/main/stdlib/typing_extensions.pyi).
+Because of the special status that `typing_extensions` holds in the typing ecosystem,
+the stubs are placed in the standard library in typeshed and distributed as
+part of the stubs bundled with individual type checkers.
+
 # Running tests
 
 Testing `typing_extensions` can be tricky because many development tools depend on

README.md

@@ -15,6 +15,10 @@ The `typing_extensions` module serves two related purposes:
 - Enable experimentation with new type system PEPs before they are accepted and
   added to the `typing` module.
 
+`typing_extensions` is treated specially by static type checkers such as
+mypy and pyright. Objects defined in `typing_extensions` are treated the same
+way as equivalent forms in `typing`.
+
 `typing_extensions` uses
 [Semantic Versioning](https://semver.org/). The
 major version will be incremented only for backwards-incompatible changes.
@@ -29,7 +33,7 @@ where `x.y` is the first version that includes all features you need.
 See [the documentation](https://typing-extensions.readthedocs.io/en/latest/#) for a
 complete listing of module contents.
 
-## Running tests
+## Contributing
 
-To run tests, navigate into the `src/` directory and run
-`test_typing_extensions.py`.
+See [CONTRIBUTING.md](https://github.com/python/typing_extensions/blob/main/CONTRIBUTING.md)
+for how to contribute to `typing_extensions`.