ext: Add an Arm section for gem5-SST README.md

Change-Id: Ia10948a836797855219aef406bf38f52d28d8e26
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/53628
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

ext/sst/README.md

@@ -57,7 +57,7 @@ the request from the SubComponent and send it to the SST memory hierarchy.
 
 See `INSTALL.md`.
 
-## Running an example simulation
+## Running an example simulation (RISCV)
 
 Downloading the built bootloader containing a Linux Kernel and a workload,
 
@@ -77,6 +77,32 @@ More information about building a bootloader containing a Linux Kernel and a
 customized workload is available at
 [https://gem5.googlesource.com/public/gem5-resources/+/refs/heads/develop/src/riscv-boot-exit-nodisk/].
 
+## Running an example simulation (Arm)
+
+Download the prebuilt bootloader and Linux Kernel with embedded initramfs and
+extract them under the $M5_PATH directory (make sure M5_PATH points to a valid
+directory):
+
+```sh
+wget http://dist.gem5.org/dist/develop/arm/aarch-sst-20211207.tar.bz2
+tar -xf aarch-sst-20211207.tar.bz2
+
+# copying bootloaders
+cp binaries/boot* $M5_PATH/binaries/
+
+# copying Linux Kernel
+cp binaries/vmlinux_exit.arm64 $M5_PATH/binaries/
+```
+
+`vmlinux_exit.arm64` contains an m5 binary, and `m5 exit` will be called upon
+the booting process reaching the early userspace.
+
+Run the simulation:
+
+```sh
+sst sst/arm_example.py
+```
+
 ## Notes
 
 - SwapReq from gem5 requires reading from memory and writing to memory.