update os-hardening readme with os_ignore_users

fixes dev-sec#542

Signed-off-by: Sebastian Gumprich <sebastian.gumprich@t-systems.com>

roles/os_hardening/README.md

@@ -223,6 +223,9 @@ We know that this is the case on Raspberry Pi.
 - `proc_mnt_options`
   - Default: `rw,nosuid,nodev,noexec,relatime,hidepid={{ hidepid_option }}`
   - Description: Mount proc with hardenized options, including `hidepid` with variable value.
+- `os_ignore_users`
+  - Default: `['vagrant', 'kitchen']`
+  - Description: Specify system accounts whose login should not be disabled and password not changed
 - `os_ignore_home_folder_users`
   - Default: `lost+found`
   - Description: specify user home folders in `/home` that shouldn't be chmodded to 700