update readme

Author: geospatial-jeff

README.md

@@ -28,5 +28,11 @@ earthengine authenticate
 Export coverage data to a BigQuery table:
 
 ```bash
-aef-export coverage BQ_DATASET_NAME BQ_TABLE_NAME
+aef-export coverage <BQ_DATASET_NAME> <BQ_TABLE_NAME>
+```
+
+Export a single image to GCS, an example image ID is `GOOGLE/SATELLITE_EMBEDDING/V1/ANNUAL/xs6bvzj41inm2e1cc`.  It is recommended to export embeddings in their quantized form (int8) to reduce storage costs.
+
+```bash
+aef-export image <GCS_BUCKET_NAME> <GCS_KEY_PREFIX> --quantize
 ```