Add deprecation policy

As suggested in AMICI-dev#2458.

Other opinions?

CHANGELOG.md

@@ -1,5 +1,15 @@
 # Changelog
 
+## Deprecation policy
+
+AMICI aims to provide a stable API for users. However, not all features can be
+maintained indefinitely. We will deprecate features in minor releases and
+where possible, issue a warning when they are used. We will keep deprecated
+features for at least six months after the release that includes the
+respective deprecation warning and then remove them earliest in the next minor
+or major release. If a deprecated feature is the source of a major bug, we may
+remove it earlier.
+
 ## v0.X Series
 
 ### v0.25.2 (2024-06-16)