Add compatibility note about HDMI on Mac Mini 2018

Add a note about the native HDMI port on the 2018 Mac Mini not working with DDC/CI to the Readme.

README.md

@@ -34,6 +34,8 @@ ExternalDisplayBrightness uses the [DDC/CI standard](https://en.wikipedia.org/wi
 
 There are some issues with certain Mac models which freeze when attempting to read the current brightness of the external display. Because of that, reading the initial brightness of the display is disabled by default, and when changing the brightness of a display for the first time, its initial brightness is set to 50%. You can enable reading the brightness from the display in the app's preferences, but do so at your own risk.
 
+Additionally, it seems the HDMI port on the 2018 Mac Mini does not work with the DDC/CI interface. Using an USB-C to HDMI adapter should work fine, though.
+
 Developing
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 The utility is a standard Xcode & Swift project without any external dependencies. You can just download/clone the repository and open *src/ExternalDisplayBrightness.xcodeproj* to start making changes.