Add diagram to "Configuration files supported by repo2docker"

[rgaiacs]

I think a diagram showing when each file might be used will be incredible valuable for "Configuration files supported by repo2docker.

[rgaiacs]

The following is a draft

graph TD;
    image[container image] --> conda[install conda]
    conda --> jupyter[install Jupyter]
    jupyter --> rstudio[install R + R Studio]
    jupyter --> apt[install system dependencies]
    rstudio --> apt
    subgraph one
    environment.yaml
    requirement.txt
    install.R
    end
    apt --> environment.yaml
    apt --> requirement.txt
    apt --> install.R

    environment.yaml --> postBuild
    requirement.txt --> postBuild
    install.R --> postBuild

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Feedback is welcome before a pull request is created.

[manics]

I think this is useful for developers of repo2docker, but perhaps goes into too much implementation detail for users.