Add qemu dependency to readme

[dhood]

riscv platform is not supported by docker buildx on ubuntu install following the docs instructions at https://docs.cartesi.io/build-dapps/requirements/#docker-and-docker-compose

Otherwise this manifests in a pretty subtle error (see https://stackoverflow.com/a/73285704 ).

This is the error for troubleshooting future user installations:

=> [server machine-core 4/6] COPY shasumfile .                                                                                                                               0.0s
 => CANCELED [server machine-core 5/6] RUN while read DEP; do wget -O $DEP; done < dependencies                                                                              14.7s
 => [server 3/4] COPY . .                                                                                                                                                     6.6s
 => ERROR [console 4/4] RUN make                                                                                                                                              0.8s
------
 > [console 4/4] RUN make:
#0 0.490 exec /bin/sh: exec format error
------
Dockerfile:6
--------------------
   4 |     WORKDIR /opt/cartesi/dapp
   5 |     COPY . .
   6 | >>> RUN make
   7 |     
--------------------
ERROR: failed to solve: process "/bin/sh -c make" did not complete successfully: exit code: 1

HTH!