docs: how to introduce functionality without breaking users

docs/project-management/deprecations.md

@@ -0,0 +1,20 @@
+# Deprecations and new functionality
+
+As kubernetes and the controller ecosystem continues to evolve,
+we will sometimes want to add new functionality or behaviour to kdp.
+
+However, as a library, we do not want to break the functionality of controllers that upgrade their kdp version,
+particularly as the coupling to kubernetes versions means that users must upgrade periodically.
+
+We will aim to follow these rules therefore:
+
+* Do not remove functionality or fundamentally change the behaviour in a way that will break users.
+
+* Bug fixes are good, but we should think about existing users even for bug fixes.
+
+* Consider introducing a new method or type instead of breaking existing functionality.
+
+* Use the go `// Deprecated:` [convention](https://go.dev/wiki/Deprecated) to discourage usage of "old" methods, fields or types.
+
+* Prefer errors at compilation time to errors at run time.  Reasonable code changes are acceptable (e.g. adding a context method).
+  Crashing at run time because a field is not populated is not acceptable.