GH-46: Clarify Drupal version in ddev config example

- update setup command to use --project-type=drupalN
- add note that N is the Drupal version number

Refs: README.md

Author: hkirsman

README.md

@@ -18,9 +18,11 @@ the subdomain of ddev.site eg if project name is example.com, then localhost
 URL will become example.com.ddev.site.
 
    ```bash
-   ddev config --project-type=drupal --docroot=web --project-name=example.com
+   ddev config --project-type=drupalN --docroot=web --project-name=example.com
    ```
 
+where N is the version of Drupal.
+
 2. Install Wunderio DDEV Drupal as a DDEV add-on and restart DDEV:
 
    ```bash