Add component description for export

src/export/README.md

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+# export
+
+The **export** component is responsible for rendering vector tiles using `osm2vectortiles.tm2source` and the **postgis** component.
+You can run **export** together with a message queue **rabbitmq** or standalone for smaller extracts where you don't need to divide the work into several parts.
+
+## Usage
+
+### Create extract
+
+Adjust the `BBOX` env var in the `export` container section of `docker-compose.yml` to fit the extract you want to create.
+
+```bash
+docker-compose run export
+```
+
+### Run as worker
+
+If you run **export** in worker mode you need to:
+
+1. Have a RabbitMQ server **rabbitmq** up and running
+2. Configure S3 access in `docker-compose.yml`
+
+```bash
+docker-compose scale export-worker=2
+```