docs: testcase

tests/assets/overpassql/README.md

@@ -2,7 +2,14 @@
 
 This folder store every QL we used in Yuheng's testing.
 
-Some was imported in before commit's message.
+Test are extracted from official documents: https://wiki.openstreetmap.org/wiki/Overpass_API/Overpass_API_by_Example#Server_Status_Queries
+
+They are only mentioned in wiki and haven't release a code repository. So we won't directly copy them in Yuheng's code repository because of license conflict.
+
+For featured overpassql you can see:
+
+(Some was imported in before commit's message.)
 
 + timezone_timeout_outmeta.overpassql: used in `OverpassAPI_export.osm`
 + timezone_timeout_outbody.overpassql: used in `OverpassAPI_without_version_export.osm`
+