clean up READMEs

each image directory should have a clear, concise README -- best current example is the [cdl-jupyter README](https://github.com/ContextLab/CDL-docker-stacks/blob/master/cdl-jupyter/README.md) (sans horrible typos).

Each README will automatically be posted with respective image on Docker Hub by GitHub action (WIP) and should have at minimum 4 parts:
1. Name (\) + brief description
2. Quick usage commands for (at minimum) 3 use cases:
1. running container directly from pre-built Docker Hub image
2. build local copy of image from Dockerfile in GitHub repo
3. Using a (tagged version of) the pre-built image on Docker Hub as a base for a new image (`FROM contextlab/...[:tag]`)
3. `build-arg`s available when running `docker build` in use case ii
4. list of pre-installed `apt`, `conda`, and `pip` packages for the given image _not present in the parent image_, plus a link to the parent image's README.