Update README.md (#26)

README.md

@@ -1,2 +1,26 @@
 # swodlr-ingest-to-sds
-swodlr-ingest-to-sds is a system for ingesting input products to on-demand SDS 
+
+A SWODLR microservice for ingesting granules from a CNM-R SNS topic to a SDS
+
+## How it works
+
+`ingest-to-sds` subscribes to a SNS topic which receives CNM-R messages from the
+PO.DAAC Cumulus pipeline which processes the PIXC, PIXCVec, and Orbit granules
+that SWODLR requires as input granules. `ingest-to-sds` then requests that the
+SDS powering SWODLR perform an ingest on the products within those CNM-R
+messages. `ingest-to-sds` will continue to track the ingest job as it proceeds
+through the SDS in order to monitor for its completion. This data is tracked
+within a DynamoDB table for later monitoring by operations/development teams.
+Errors that result from ingestion will also be cataloged within that table
+for quick mass troubleshooting/diagnostics.
+
+`ingest-to-sds` is a purely internal SWODLR service and is not part of any user
+request pipeline. `ingest-to-sds` also operates as a "best-effort" service.
+Because `raster-create` can perform its own ingest process for required granules
+via its Preflight process, `ingest-to-sds` is simply a service to help bring
+down latencies for `raster-create` by preloading the granule database on the
+SDS.
+
+`ingest-to-sds` is ***NOT*** a critical SWODLR service and occasional errors
+that occur during its operation should ***NOT*** be treated with high priority
+**unless occuring with unusually high frequency**