Add mquery to readme

[olof-nord]

After asking @estesp if he wanted to support riscv, support was recently added.

This tool comes in handy to determine which architectures a docker image support, something which I see myself doing quite often with riscv.

[estesp]

The default mquery "latest" image on DockerHub has been updated with the riscv support added in the linked issue above.

Verification:

$ docker run --rm mplatform/mquery:v0.5.0 mplatform/mquery:latest
Image: mplatform/mquery:latest (digest: sha256:d0989420b6f0d2b929fd9355f15c767f62d0e9a72cdf999d1eb16e6073782c71)
 * Manifest List: Yes (Image type: application/vnd.docker.distribution.manifest.list.v2+json)
 * Supported platforms:
   - linux/ppc64le
   - linux/amd64
   - linux/386
   - linux/s390x
   - linux/riscv64
   - linux/arm64/v8
   - linux/arm/v7
   - linux/arm/v6
   - windows/amd64:10.0.17763.2300
   - windows/amd64:10.0.14393.4770

[carlosedp]

Awesome, thanks @olof-nord and @estesp !