feat: HOWTO backwards compat in tmpl files

docs/maintainers/upgrade.md

@@ -56,6 +56,14 @@ environment:
     - NEW_ENV_VAR=${NEW_ENV_VAR:-false}
 ```
 
+Another approach is to set this default within the configuration where you
+reference the environment variable. In `.tmpl` files, this generally works out
+something like this:
+
+```
+my_config_option = {{ or (env "NEW_ENV_VAR") "false" }}
+```
+
 Sometimes it is not appropriate to provide a default and it is important that
 other Co-op Cloud operators make an informed choice about the setting of an
 environment variable. In this case, please consider leaving a note about your