README should mention needing docker buildx plugin

[dwilliss]

Desired Behavior

The build.sh script should work
The README should mention another prerequisite.

Contrast to Current Behavior

I was trying to run the build.sh script and getting an error saying that buildx is not a docker command.
I was able to find instructions on how to install this plugin:

1. wget https://github.com/docker/buildx/releases/download/v0.4.1/buildx-v0.4.1.linux-amd64
2. chmod a+x buildx-v0.4.1.linux-amd64
3. mkdir -p ~/.docker/cli-plugins
4. mv buildx-v0.4.1.linux-amd64 ~/.docker/cli-plugins/docker-buildx

Although these instructions are old and the current version (for now) is 0.11.2

Required Changes

Adding a section to the README along with the other prerequisites would help solve this issue.

Discussion: Benefits and Drawbacks

No response