Python: Add change-note

Author: RasmusWL

python/ql/src/change-notes/2022-04-05-add-xxe-and-xmlbomb.md

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+---
+category: newQuery
+---
+* "XML external entity expansion" (`py/xxe`). Results will appear by default. This query was based on [an experimental query by @jorgectf](https://github.com/github/codeql/pull/6112).
+* "XML internal entity expansion" (`py/xml-bomb`). Results will appear by default. This query was based on [an experimental query by @jorgectf](https://github.com/github/codeql/pull/6112).