Update and reorganize documentation.

* doc/Open API document 1.2.pdf: Remove, after discussion with Eric.

* doc/Model Database Tables.jpg: Rename to...

* doc/schema-diagram.jpg: ...here.

* doc/Database Properties from file.pdf: Move to...

* INSTALL.md: ...here, and reformat in Markdown.

* README.md: Refer to INSTALL.md.

* API.md: Describe current status better; refer to schema diagram.