nit on README

Author: jaimejim

README.md

@@ -25,6 +25,12 @@ Formatted text and HTML versions of the draft can be built using `make`.
 $ make
 ```
 
+You might want to run first
+
+```sh
+make update-files
+```
+
 Command line usage requires that you have the necessary software installed.  See
 [the instructions](https://github.com/martinthomson/i-d-template/blob/main/doc/SETUP.md).