Python: Add change-note

I went with `minorAnalysis` instead of `majorAnalysis`, since I don't
think the impact of this change will be major (but that's just my gut
feeling).

Author: RasmusWL

python/ql/lib/change-notes/2022-02-04-improve-field-flow.md

@@ -0,0 +1,5 @@
+---
+category: minorAnalysis
+---
+* Improved analysis of attributes for data-flow and taint tracking queries, so `getattr`/`setattr` are supported, and a write to an attribute properly stops flow for the old value in that attribute.
+* Added post-update nodes (`DataFlow::PostUpdateNode`) for arguments in calls that can't be resolved.