README: riscv: Add documentation for new platform

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

README.md

@@ -24,16 +24,6 @@ This project was originally developed using
 currently support resetting the virtio block device it is not possible to boot
 all the way into the OS.
 
-## Building
-
-To compile:
-
-cargo build --release --target x86_64-unknown-none.json -Zbuild-std=core,alloc -Zbuild-std-features=compiler-builtins-mem
-
-The result will be in:
-
-target/x86_64-unknown-none/release/hypervisor-fw
-
 ## Features
 
 * virtio (PCI) block support
@@ -44,15 +34,31 @@ target/x86_64-unknown-none/release/hypervisor-fw
 * PE32+ loader
 * Minimal EFI environment (sufficient to boot shim + GRUB2 as used by Ubuntu)
 
-## Running
+## x86-64 Support
+
+### Building
+
+To compile:
+
+```
+cargo build --release --target x86_64-unknown-none.json -Zbuild-std=core,alloc -Zbuild-std-features=compiler-builtins-mem
+```
+
+The result will be in:
+
+```
+target/x86_64-unknown-none/release/hypervisor-fw
+```
+
+### Running
 
 Works with Cloud Hypervisor and QEMU via their PVH loaders as an alternative to
 the Linux kernel.
 
 Cloud Hypervisor and QEMU are currently the primary development targets for the
 firmware although support for other VMMs will be considered.
 
-### Cloud Hypervisor
+#### Cloud Hypervisor
 
 As per [getting
 started](https://github.com/cloud-hypervisor/cloud-hypervisor/blob/master/README.md#2-getting-started)
@@ -73,7 +79,7 @@ $ ./cloud-hypervisor/target/release/cloud-hypervisor \
 $ popd
 ```
 
-### QEMU
+#### QEMU
 
 Use the QEMU `-kernel` parameter to specify the path to the firmware.
 
@@ -88,6 +94,39 @@ $ qemu-system-x86_64 -machine q35,accel=kvm -cpu host,-vmx -m 1G\
     -device virtio-blk-pci,drive=os,disable-legacy=on
 ```
 
+## RISC-V Support
+
+Experimental RISC-V support is avaiable. This is currently designed to run as a
+payload from OpenSBI under QEMU virt. It is expected wider platform support
+will become available in the future.
+
+### Building
+
+To compile:
+
+```
+cargo build --release --target riscv64gcv-unknown-none-elf.json -Zbuild-std=core,alloc -Zbuild-std-features=compiler-builtins-mem
+```
+
+The result will be in:
+
+```
+target/riscv64gcv-unknown-none-elf/release/hypervisor-fw
+```
+
+### Running
+
+Currently only QEMU has been tested.
+
+#### QEMU
+
+```
+$ qemu-system-riscv64  -M virt -cpu rv64 -smp 1 -m 1024 \
+    -nographic -kernel target/riscv64gcv-unknown-none-elf/release/hypervisor-fw \
+    -drive id=mydrive,file=root.img,format=raw \
+    -device virtio-blk-pci,drive=mydrive,disable-legacy=on
+```
+
 ## Testing
 
 "cargo test" needs disk images from make-test-disks.sh