Improve Documentation

[johndgiese]

• [x] Add an "Installation" section; we can install using pip install dicom-standard. Also include the curl or wget commands needed to pull down either the latest version of the JSON or a particular pinned version.
• [x] Add a section which describes our philosophy for the format of the JSON files; in particular, that it is likely that the JSON will need to get massaged for other use-cases. Mention that this was originally for our DICOM standard browser (and link to it). Then discuss how our philosophy has changed to be more general. See https://github.com/innolitics/dicom-standard/issues/24#issuecomment-591579455 for a good starting place!
• [x] Take a pass through the existing sections and clean them up; prune them out as much as possible
• [x] Discuss how we handle issues with the DICOM standard
• [x] Discuss how often/when we update the JSON files
• [ ] Explain how we handle functional group macros, and why they are a weird exception. Maybe we could include it in a list of the processing steps we handle on behalf of users of the library, along with expanding macros and the various DICOM standard inconsistencies we correct for.

[johndgiese]

We should include a nice diagram of the relational model of the JSON files (e.g., like we do for most of our application database schema discussions).