Added custom sort map example

pkg/yqlib/doc/operators/headers/sort-keys.md

@@ -11,3 +11,6 @@ diff file1.yml file2.yml
 ```
 
 Note that `yq` does not yet consider anchors when sorting by keys - this may result in invalid yaml documents if your are using merge anchors.
+
+For more advanced sorting, using `to_entries` to convert the map to an array, then sort/process the array as you like (e.g. using `sort_by`) and convert back to a map using `from_entries`.
+See [here](https://mikefarah.gitbook.io/yq/operators/entries#custom-sort-map-keys) for an example.