Update readme

README.md

@@ -1,4 +1,4 @@
-# Amateurfunk
+# Amateurfunk-Anki
 
 Download the German amateur-radio exam question catalog from the
 Bundesnetzagentur and turn it into Anki decks.
@@ -17,6 +17,25 @@ Output: three `.apkg` files under `anki/`, one per Prüfungsteil
 (Technische / Betriebliche / Vorschriften). Import them into Anki —
 re-importing a newer build preserves your review history.
 
+## Exam question source
+
+The catalog is published by the Bundesnetzagentur (BNetzA), the German
+federal regulator for telecommunications. Current edition:
+**3. Auflage, März 2024** (issued 2024-03-20, ~1750 questions across
+license classes N, E, A).
+
+- Landing page: <https://www.bnetza.de/amateurfunk-fragenkatalog>
+- Machine-readable ZIP (what we download):
+  [`PruefungsfragenZIP.zip`](https://www.bundesnetzagentur.de/SharedDocs/Downloads/DE/Sachgebiete/Telekommunikation/Unternehmen_Institutionen/Frequenzen/Amateurfunk/Fragenkatalog/PruefungsfragenZIP.zip?__blob=publicationFile)
+  — contains the JSON question tree, the `svgs/` figure folder, and a
+  `README.txt` with the official Quellenvermerk.
+- Human-readable PDF (not used by this pipeline):
+  [`Pruefungsfragen.pdf`](https://www.bundesnetzagentur.de/SharedDocs/Downloads/DE/Sachgebiete/Telekommunikation/Unternehmen_Institutionen/Frequenzen/Amateurfunk/Fragenkatalog/Pruefungsfragen.pdf?__blob=publicationFile)
+
+BNetzA replaces the file in place across editions, so the URL is
+stable; the fetcher detects updates via the HTTP `Last-Modified`
+header.
+
 ## Requirements
 
 Python 3.11+, standard library only. No third-party dependencies.