Add comment when loading columnar.so in citus

src/backend/distributed/shared_library_init.c

@@ -347,7 +347,11 @@ _PG_init(void)
 		DoInitialCleanup();
 	}
 
-
+	/*
+ 	* For convenience and backwards compatibility, we avoid users having to add both
+ 	* citus and columnar to shared_preload_libraries by loading columnar.so as part of
+ 	* loading citus.so.
+ 	*/
 	load_file("columnar.so", false);
 }