Document deploying Coder Desktop for macOS over MDM

There's a few things that MDM admins need to do to get Coder Desktop working over MDM, these include:
- Ensuring the package installer runs as root (this might always be the case in MDM, will need to check)
- Pre-approving the Network Extension bundle. This is done by supplying the extension identifier (`com.coder.Coder-Desktop.VPN`)
- Disabling the built-in updater. This is done by forcibly setting the value of `disableUpdater` in the app's userdefaults plist. Support for this was added in #219.