Improve Documentation

[johndgiese]

• 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.
• 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 Add "id" property to JSON files to act as a foreign key #24 (comment) for a good starting place!
• Take a pass through the existing sections and clean them up; prune them out as much as possible
• Discuss how we handle issues with the DICOM standard
• 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).