Update README to add a link explaining config overrides.

Author: geerlingguy

README.md

@@ -42,7 +42,7 @@ Full Drupal VM documentation is available at http://docs.drupalvm.com/
 
 There are a couple places where you can customize the VM for your needs:
 
-  - `config.yml`: Contains variables like the VM domain name and IP address, PHP and MySQL configuration, etc.
+  - `config.yml`: Override any of the default VM configuration from `default.config.yml`; customize almost any aspect of any software installed in the VM (more about [overriding configurations](http://docs.drupalvm.com/en/latest/other/overriding-configurations/).
   - `drupal.make.yml`: Contains configuration for the Drupal core version, modules, and patches that will be downloaded on Drupal's initial installation (more about [Drush make files](https://www.drupal.org/node/1432374)).
 
 If you want to switch from Drupal 8 (default) to Drupal 7 on the initial install, do the following: