Document deploying Coder Desktop for macOS over MDM

[ethanndickson]

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 feat: support disabling the built-in updater #219.