Update sources.sql with a comment as to date format

schema/sources.sql

@@ -6,7 +6,7 @@ CREATE TABLE sources (
   sources_short_name TEXT NOT NULL UNIQUE,
   sources_abbrev TEXT NOT NULL UNIQUE,
   sources_descr TEXT NOT NULL UNIQUE,
-  release_date TEXT NOT NULL,
+  release_date TEXT NOT NULL, -- in YYYY-MM-DD format
   first_party BOOLEAN NOT NULL,
   ogl_copyright_block TEXT NOT NULL
 );