docs: show dry run

Author: steveoh

README.md

@@ -37,6 +37,12 @@ To run the setup:
 ansible-playbook -K _all.yaml
 ```
 
+To see what has changed since the last run:
+
+```bash
+ansible-playbook -K _all.yaml --check --diff
+```
+
 You will be prompted for your password, so that administrative-level software can be installed. _**You must be a sudoer to run these scripts, otherwise the installation process will fail.**_ You can also run individual files if you'd prefer to take more control over what's executed.
 
 Since core OS packages are upgraded, it is safest to reboot the PC/VM after running these scripts. At a bare minimum, many UI shell customizations done here will require you to log out and log back in.