ubuntu: mention riscv64 architecture in README.md

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

Author: xypron

ubuntu/README.md

@@ -80,8 +80,8 @@ kernel on deploy.
 
 ### Building different architectures
 
-By default, images are produces for amd64. You can build for arm64 as well if
-you specify the `architecture` parameter:
+By default, images are produces for amd64. You can build for arm64 or riscv64
+as well if you specify the `architecture` parameter:
 
 ```shell
 packer init .