Database schema reorganization

[caparker]

I propose a slight reorganization to the structure of the files in this project to facilitate more meta data about the database tables and views.

Right now we have

openaqdb
-- tables
-- -- countries.sql

Where the table definitions are organized by table name, one table per file. And

openaqdb
-- idempodent
-- -- views.sql 

Where all views are in one file

I propose a more verbose structure that would allow for README.md files that would contain schema details as well as information as to why the table/view exists.

For example

openaqdb (to not change too much)
-- tables
-- -- countries
-- -- -- README.md
-- -- -- countries.sql

Or something like that. We could also split up all the views and functions as well and have readme files for all of those.

[russbiggs]

I like this structure with individual readmes per view