OvmfPkg: README: Add documentation for Standalone MM on OVMF

This update introduces a new section in the README file, detailing the
design considerations of the recently onboarded Standalone MM platform in
OVMF.

The section is intended for future reference and should be revised
accordingly if updates to the Standalone MM framework render these
limitations obsolete.

Signed-off-by: Kun Qin <kun.qin@microsoft.com>

OvmfPkg/README

@@ -214,6 +214,47 @@ can be used on Windows.
 
   -global ICH9-LPC.disable_s3=1 \
 
+=== Standalone MM Support ===
+
+Standalone MM (Management Mode) in UEFI is a secure execution environment
+provided by the CPU and related silicon, designed to improve security and
+portability compared to Traditional MM. It operates independently of the DXE
+(Driver Execution Environment) phase, ensuring better isolation and reducing
+vulnerabilities.
+
+Standalone MM leverages the same hardware capabilities as Traditional MM but
+revises the software model to address security challenges. It uses Management
+Mode RAM (MMRAM) for executing drivers and protocols securely, and its
+initialization and runtime phases are distinct from Traditional MM.
+
+Due to the nature of Standalone MM, there are some limitations, requirements and
+considerations when using it in OVMF:
+
+* Standalone MM is only supported in X64 mode and is not available in IA32 mode.
+* The Standalone MM driver must be built with `-D STANDALONE_MM_ENABLE` flag.
+* For X64, Standalone MM does not currently support S3 resume or LockBox
+  functionality. While LockBox functionality could be supported in the future
+  with unblock memory enabled during the DXE phase, S3 resume support is not
+  planned.
+* On OVMF, Standalone MM does not support CPU hotplugging at this time; however,
+  this feature may be enabled in the future.
+* Similar to SMM, Standalone MM requires a pflash-backed variable store.
+* Standalone MM framework copies the entire Firmware Volume (FV) containing the
+  Standalone MM core into MMRAM. As a result, MMRAM must have sufficient
+  capacity to accommodate this operation alongside the runtime-loaded drivers.
+
+* Example QEMU launching command for Standalone MM based Q35 machine type:
+
+  $ qemu-system-x86_64 \
+  -debugcon stdio \
+  -smp 4 -cpu IvyBridge,+rdrand \
+  -machine q35,smm=on --accel tcg,thread=single \
+  -global driver=cfi.pflash01,property=secure,value=on \
+  -drive if=pflash,format=raw,unit=0,file=OVMF_CODE.fd,readonly=on \
+  -drive if=pflash,format=raw,unit=1,file=OVMF_VARS.fd \
+  -global ICH9-LPC.disable_s3=1 \
+  -global mch.extended-tseg-mbytes=32
+
 === Network Support ===
 
 OVMF provides a UEFI network stack by default. Its lowest level driver is the