Improve project scope defined in README

[parente]

1. Make clearer what libraries are known to work with dashboard server at the moment in a one-click fashion. (They're in the README, but a bit buried.)
2. Explain the three-ish ways that libraries use to get JS to the web page of a notebook and how they apply in the dashboard server: a) cell output, b) bundled, c) pre-webpacked.
3. Explain what a user / dashboard server provider needs to do to add new support in each of the cases (roughly: a) nothing, b) writing a bundler / creating a manual bundle, c) submitting a PR here or forking)

References: jupyter/dashboards#275, jupyter/dashboards#277, #300, #301, #188, #135

/cc @DTAIEB

[parente]

I'll draft a PR during my plane ride today and ping others to comment.