Add documentation for using with mkdocs-material dockerfile

[bavedarnow]

When installing the plugin in the mkdocs-material dockerfile, a few more dependencies were needed because it uses an alpine base image. This would be good to put in the readme

FROM squidfunk/mkdocs-material
RUN apk --no-cache add msttcorefonts-installer fontconfig && \
    update-ms-fonts && \
    fc-cache -f && \
    pip install mkdocs-diagrams graphviz jinja2

See mingrammer/diagrams#231 for more info