docs: changelog entry for the read-only proxy method contract (#794)

CHANGELOG.md

@@ -1,5 +1,13 @@
 # Changelog
 
+- The read-only query proxies state their method contract. `/api/opensky`,
+  `/api/adsblol/mil` and `/api/route` answered any verb with the same body
+  while ignoring the request; they now answer `405 Method Not Allowed` to
+  anything but `GET`, the way `/api/geocode` and the other query proxies
+  already did, and declare `Allow: GET` with it the way `/api/radio/stations`
+  does. The refusal precedes rate limiting and any upstream call
+  (daikaginza, #794).
+
 - Public Overpass instances are no longer used by default. Street Traffic
   roads come from TomTom flow tiles, OpenFreeMap vector tiles, or both, chosen
   on the layer row (TomTom / OSM / Hybrid) or with `?trafficRoads=`. With a