docs: Add notes about UEFI features

Document that only UEFI booting is supported, the current status of
Secure Boot, and that the shell and networking are disabled.

Signed-off-by: Tim Crawford <[email protected]>

Author: crawfxrd

docs/uefi.md

@@ -0,0 +1,23 @@
+# UEFI
+
+System76 uses [EDK2](https://github.com/tianocore/edk2) to implement UEFI.
+
+[coreboot](https://coreboot.org/) is used for Platform Initialization (PI).
+
+## Booting
+
+System76 Open Firmware only supports UEFI booting. Legacy BIOS-MBR booting is
+not supported. `\EFI\BOOT\BOOTX64.EFI` must exist on the EFI System Partition
+to be considered valid.
+
+Network functionality is disabled. Native PXE booting is not supported.
+
+### Secure Boot
+
+Support for Secure Boot is present, but is not configurable through the
+firmware UI. Systems will boot in Setup Mode with Secure Boot disabled.
+
+## Shell
+
+The internal UEFI shell is disabled. A separate binary on a bootable drive
+must be used to access the shell environment.