Add a note about old Makefiles

Author: martinthomson

doc/TIPS.md

@@ -49,13 +49,15 @@ them, or builds will fail in CI or for other people.
 
 Set `ID_TEMPLATE_HOME` in your environment to a common location
 (such as a checkout of this repository) and the `Makefile` will create
-a symlink for you.
+a symlink for you[^old].
 
 ```sh
 $ git clone https://github.com/martinthomson/i-d-template i-d-template
 $ echo 'export ID_TEMPLATE_HOME="'"$(pwd)"'/i-d-template"' >> ~/.profile
 ```
 
+[^old]: Old versions of the `Makefile` don't, so you might need to update.
+
 
 ## When Creating Pull Requests on Another Repository