Python: add change note

Author: asgerf

python/ql/lib/change-notes/api-graph-api.md

@@ -0,0 +1,10 @@
+---
+category: library
+---
+
+* The documentation of API graphs (the `API` module) has been expanded, and some of the members predicates of `API::Node`
+  have been renamed as follows:
+  - `getAnImmediateUse` -> `asSource`
+  - `getARhs` -> `asSink`
+  - `getAUse` -> `getAValueReachableFromSource`
+  - `getAValueReachingRhs` -> `getAValueReachingSink`