Introduce a backwards comp policy (#474)

* Introduce a backwards comp policy

* Tweak doc some more

.github/SECURITY.md

@@ -1,5 +1,19 @@
-## Security contact information
+# Security Policy
 
-To report a security vulnerability, please use the
-[Tidelift security contact](https://tidelift.com/security).
+
+## Versioning and Backwards Compatibility
+
+_cattrs_ follows [*CalVer*](https://calver.org) and we only support the latest PyPI version, due to limited resources.
+We aim to support all Python versions that are not end-of-life; older versions will be dropped to ease the maintenance burden.
+
+Our goal is to never undertake major breaking changes (the kinds that would necessitate a v2 if we were following SemVer).
+Minor breaking changes may be undertaken to improve the developer experience and robustness of the library.
+All breaking changes are prominently called out in the changelog, alongside any customization steps that may be used to restore previous behavior, when applicable.
+
+APIs may be marked as provisional.
+These are not guaranteed to be stable and may change or be removed without prior notice.
+
+## Security Contact Information
+
+To report a security vulnerability, please use the [Tidelift security contact](https://tidelift.com/security).
 Tidelift will coordinate the fix and disclosure.

HISTORY.md

@@ -2,10 +2,18 @@
 ```{currentmodule} cattrs
 ```
 
+This project adheres to [Calendar Versioning](https://calver.org/).
+The first number of the version is the year.
+The second number is incremented with each release, starting at 1 for each year.
+The third number is for emergencies when we need to start branches for older releases.
+
+Our backwards-compatibility policy can be found [here](https://github.com/python-attrs/cattrs/blob/main/.github/SECURITY.md).
+
 ## 24.1.0 (UNRELEASED)
 
 - **Potentially breaking**: Unstructuring hooks for `typing.Any` are consistent now: values are unstructured using their runtime type.
   Previously this behavior was underspecified and inconsistent, but followed this rule in the majority of cases.
+  Reverting old behavior is very dependent on the actual case; ask on the issue tracker if in doubt.
   ([#473](https://github.com/python-attrs/cattrs/pull/473))
 - Introduce {meth}`BaseConverter.get_structure_hook` and {meth}`BaseConverter.get_unstructure_hook` methods.
   ([#432](https://github.com/python-attrs/cattrs/issues/432) [#472](https://github.com/python-attrs/cattrs/pull/472))