Extend docs intro (#168)

Author: JelleZijlstra

doc/index.rst

@@ -19,17 +19,33 @@ repository. If the PEP is accepted, the feature will then be added to the
 affected ``typing_extensions`` has been rejected so far, so we haven't yet
 figured out how to deal with that possibility.
 
+Bugfixes and new typing features that don't require a PEP may be added to
+``typing_extensions`` once they are merged into CPython's main branch.
+
+Versioning and backwards compatibility
+--------------------------------------
+
 Starting with version 4.0.0, ``typing_extensions`` uses
-`Semantic Versioning <https://semver.org>`_. The
-major version is incremented for all backwards-incompatible changes.
+`Semantic Versioning <https://semver.org>`_. A changelog is
+maintained `on GitHub <https://github.com/python/typing_extensions/blob/main/CHANGELOG.md>`_.
+
+The major version is incremented for all backwards-incompatible changes.
 Therefore, it's safe to depend
 on ``typing_extensions`` like this: ``typing_extensions >=x.y, <(x+1)``,
 where ``x.y`` is the first version that includes all features you need.
 In view of the wide usage of ``typing_extensions`` across the ecosystem,
 we are highly hesitant to break backwards compatibility, and we do not
 expect to increase the major version number in the foreseeable future.
 
-``typing_extensions`` supports Python versions 3.7 and higher. In the future,
+Before version 4.0.0, the versioning scheme loosely followed the Python
+version from which features were backported; for example,
+``typing_extensions`` 3.10.0.0 was meant to reflect ``typing`` as of
+Python 3.10.0. During this period, no changelog was maintained.
+
+Python version support
+----------------------
+
+``typing_extensions`` currently supports Python versions 3.7 and higher. In the future,
 support for older Python versions will be dropped some time after that version
 reaches end of life.