[README.md] external diff command documentation update

Author: chrissimpkins

README.md

@@ -162,7 +162,7 @@ By default, fdiff performs diffs with Python source.  If you run into performanc
 For example, you may run the `diff -u` command on GNU/Linux or macOS like this:
 
 ```
-$ fdiff --external
+$ fdiff --external="diff -u" [PRE-FONT FILE PATH] [POST-FONT FILE PATH]
 ```
 
 fdiff supports external diff tool with colored formatting and OpenType table filtering.  The context line, head, and tail options are not supported with the use of external diff tools.