docs(changelog): move the entry into a changelog.d fragment

Main now takes pending entries as changelog.d/<pr>-<slug>.md
files (#787), so the entry leaves CHANGELOG.md for changelog.d/795-opensky-compress-cached-body.md.

changelog.d/795-opensky-compress-cached-body.md

@@ -0,0 +1,7 @@
+- The live flight snapshot is compressed on the wire. `/api/flights` gzips its
+  cached global snapshot and its regional adsb.lol fallback once per upstream
+  refresh and serves that frame to clients offering gzip — roughly 860 KB
+  becomes roughly 258 KB — while a client that offers nothing receives the same
+  bytes uncompressed. Both answers carry `Vary: Accept-Encoding`, and a response
+  the proxy does not cache, such as an auth failure, is never sent under a
+  cached snapshot's encoding (daikaginza, #795).