Added basic database structure

README.md

-# OBM-Buildings Database Management
+# `OBM Buildings` Database Management
 
 This repository handles the structure and migrations in the `obm_buildings` database.
 
 The database is described through sql files in the `migrations/` directory.
 
+## Database tables
+
+####  `obm_buildings` - Main OBM Tiles information
+
+| column_name | data_type                |
+|-------------|--------------------------|
+| osm_id      | integer (primary key)    |
+| geometry    | geometry (Polygon, 4326) |
+
+#### `migrations` - Information about database migrations
+
+Created and managed by [inyuu](https://git.gfz-potsdam.de/dynamicexposure/openbuildingmap/inyuu):
+
+| column_name | data_type    |
+|-------------|--------------|
+| name        | varchar(100) |
+| hash        | varchar(60)  |
+| executed_at | timestamp    |
+
 ## More information
 
 * [Database versioning best practices](https://enterprisecraftsmanship.com/posts/database-versioning-best-practices/)

migrations/00001_create-main-table.sql

+-- Create the `obm_buildings` table
+
+CREATE TABLE public.obm_buildings
+(
+    osm_id integer PRIMARY KEY
+);
+
+ALTER TABLE public.obm_buildings
+    OWNER to postgres;

migrations/00002_create-postgis-extension.sql

+-- Create PostGIS extension
+
+CREATE EXTENSION IF NOT EXISTS postgis;

migrations/00003_add_column_geometry.sql

+ALTER TABLE public.obm_buildings
+    ADD COLUMN IF NOT EXISTS geometry geometry (Polygon, 4326);