Merge pull request #1271 from marcomaas1990/patch-1

Added riscv64 build description

README.md

@@ -28,6 +28,7 @@ cargo xtask build --arch x86_64
 
 On completion, the script will print the path of `libhermit.a`.
 If you want to build the kernel for aarch64, please replace `x86_64` by `aarch64`.
+If you want to build the kernel for riscv64, please use `riscv64`. 
 
 ### Control the kernel messages verbosity