docs: clarify supported Python versions

Author: Rogdham

CHANGELOG.md

@@ -13,6 +13,7 @@ adheres to [Semantic Versioning](https://semver.org/).
 
 - Add tests for CPython 3.10 and PyPy 3.8
 - Use CPython 3.10 for misc. tests
+- Clarify which Python versions are supported in readme
 
 ## [0.3.0] - 2021-11-07
 

README.md

@@ -172,3 +172,17 @@ $ xz --block-list 16M,32M,8M,42M file  # specific size for each block
 ```sh
 $ pixz file
 ```
+
+### Python version support
+
+As a general rule, all Python versions that are both [released and still officially
+supported][python-versions] are supported by `python-xz` and tested against (both
+CPython and PyPy implementations).
+
+Moreover, Python 3.6 is currently supported as well, but may be dropped in future
+releases.
+
+If you have other use cases or find issues with some Python versions, feel free to
+[open a ticket](https://github.com/Rogdham/python-xz/issues/new)!
+
+[python-versions]: https://devguide.python.org/#status-of-python-branches