Document that we conform to SPEC0 version support. (#487)

(Finally) closes 
- #362

as per the definition of done in @dstansby's last comment.

Think this is fairly uncontroversial (the decision itself is already
logged in #362).

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: David Stansby <[email protected]>

Author: 3 people

CONTRIBUTING.md

@@ -53,6 +53,14 @@ cookiecutter /path/to/your/checkout/of/python-tooling --checkout <branch-name>
 You can omit the `--checkout` option if you're already on the
 branch you want to test.
 
+#### Python version support
+
+[![SPEC 0 — Minimum Supported Dependencies](https://img.shields.io/badge/SPEC-0-green?labelColor=%23004811&color=%235CA038)](https://scientific-python.org/specs/spec-0000/)
+
+We follow Scientific Python's [SPEC 0](https://scientific-python.org/specs/spec-0000/) policy.
+Notably this means our template will support [Python versions for three years after their release](https://scientific-python.org/specs/spec-0000/#support-window), and changes to the template are required to pass tests in all such python versions.
+(See [the CI checks](./.github/workflows/test.yml) for precise details.)
+
 ### Developing the recommended tooling pages
 
 Pages all live in the `docs/pages` sub-directory, and are written in markdown.