[DOCS] Screen resolution guide.

Hopefully save users some Googling and troubleshooting with these instructions.

Author: geckolinux

docs/guide-screen-resolution.md

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+## How to increase screen resolution for macOS-Simple-KVM
+
+_(Thanks to [passthroughpo.st](https://passthroughpo.st/new-and-improved-mac-os-tutorial-part-1-the-basics/) and [urcomputertechnics.com](http://urcomputertechnics.com/how-to-mount-efi-partition-on-macos-mojave/) for the tips.)_
+
+1. In the macOS Finder, look for **EFI** in the left bar under **Volumes**. If it isn't visible you will have to mount it:
+ - Open the macOS Terminal and type `diskutil list` and look for the disk/partition location of the EFI. (There may be more than one.)
+ - Type `sudo diskutil mount diskYsZ`, using the disk/partition location name where you see EFI.
+ - The **EFI** partition will appear in the left Finder bar under **Volumes**.
+ - If you don't see anything in that volume after browsing to it, try the other ones that you found in `diskutil`.
+2. In the **EFI** volume, go into the `Clover` directory and open the `config.plist` file in the macOS text editor.
+3. There should be a section of the file that looks like this:
+
+```````````````````
+<key>ScreenResolution</key>
+<string>1280x720</string>
+```````````````````
+
+ - Edit that to your preferred screen resolution.
+ - Some odd/intermediate resolutions like 1366×768 may not work well. Try to stick to more common 16:9, 16:10, and 4:3 form factors.