Add section concerning aliases, shell functions

closes #870

README.md

@@ -418,6 +418,14 @@ use a character class with a single hyphen character:
 > fd '[-]pattern'
 ```
 
+### "Command not found" for `alias`es or shell functions
+
+Shell `alias`es and shell functions can not be used for command execution via `fd -x` or
+`fd -X`. In `zsh`, you can make the alias global via `alias -g myalias="…"`. In `bash`,
+you can use `export -f my_function` to make available to child processes. You would still
+need to call `fd -x bash -c 'my_function "$1"' bash`. For other use cases or shells, use
+a (temporary) shell script.
+
 ## Integration with other programs
 
 ### Using fd with `fzf`