macOS: Added ready-to-deploy example mobileconfig file to block all removable storage devices using Device Control on Microsoft Defender

[janparttimaa]

This pull request is containing following changes:

File: "Microsoft Defender - Device Control - Block All Removable Storage Devices.mobileconfig"

Rationale: The purpose of this example policy is to help system administrators in entirely disabling the use of mass storage devices, thereby reducing the system's attack surface and hence enforcing users to use appropriate storage locations e.g. SharePoint Online. With this example, admins can easily block these devices.

Updated "README.md"

Rationale: The purpose of the updated readme -file is to provide more information of file "Microsoft Defender - Device Control - Block All Removable Storage Devices.mobileconfig" and what needs to be done before deploying the custom policy. This readme also contain screenshot.