Add component description for import-sql

src/import-sql/README.md

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+# import-sql
+
+The **import-sql** component is responsible to import the SQL used for the different layers. It will also generate SQL code for different classifications and code to detect changed tiles and table management commands for different layers. 
+
+## Usage
+
+### Docker
+
+This needs to run after **import-external** and **import-osm**. Only after **import-sql** has been run can you start generating vector tiles.
+
+```
+docker-compose run import-sql
+```
+