docs: add basic use (#34)

Signed-off-by: Miroslav Bajtoš <[email protected]>
Co-authored-by: Julian Gruber <[email protected]>

README.md

@@ -4,6 +4,59 @@ A lightweight IPNI node mapping Filecoin PieceCID → payload block CID.
 
 - [Design doc](./docs/design.md)
 
+## Basic use
+
+Note: this will change soon, see
+https://github.com/filecoin-station/piece-indexer/issues/33
+
+## `GET /sample/{provider-id}/{piece-cid}
+
+Sample a set of multihashes ingested by IPNI for a given ContextID, matching the
+route eventually to be exposed by an IPNI reverse index.
+
+_This is an alternative implementation of the IPNI Reverse Index as specified in
+[xedni/openapi.yaml](https://github.com/ipni/xedni/blob/526f90f5a6001cb50b52e6376f8877163f8018af/openapi.yaml)._
+
+Parameters:
+
+- `provider-id` - the peer ID of the storage provider (index publisher)
+- `piece-cid` - the Filecoin deal's PieceCID as advertised by the provider in
+  Graphsync retrieval metadata
+
+Response:
+
+- `samples` - a list of exactly one payload block CID contained inside the piece
+  identified by the requested PieceCID.
+
+Example:
+
+https://pix.filspark.com/sample/12D3KooWHKeaNCnYByQUMS2n5PAZ1KZ9xKXqsb4bhpxVJ6bBJg5V/baga6ea4seaqlwzed5tgjtyhrugjziutzthx2wrympvsuqhfngwdwqzvosuchmja
+
+```json
+{
+  "samples": ["bafkreigrnnl64xuevvkhknbhrcqzbdvvmqnchp7ae2a4ulninsjoc5svoq"]
+}
+```
+
+## `GET /ingestion-status/{provider-id}`
+
+Return the index status for the given provider.
+
+Example:
+
+https://pix.filspark.com/ingestion-status/12D3KooWHKeaNCnYByQUMS2n5PAZ1KZ9xKXqsb4bhpxVJ6bBJg5V
+
+```json
+{
+  "providerId": "12D3KooWHKeaNCnYByQUMS2n5PAZ1KZ9xKXqsb4bhpxVJ6bBJg5V",
+  "ingestionStatus": "All advertisements from baguqeeralhduow57bhqo5zgiwe6swgjrqw2nuckvnch6nlylaensfzf3bfyq to the end of the chain were processed.",
+  "lastHeadWalkedFrom": "baguqeeralhduow57bhqo5zgiwe6swgjrqw2nuckvnch6nlylaensfzf3bfyq",
+  "adsMissingPieceCID": 0,
+  "entriesNotRetrievable": 0,
+  "piecesIndexed": 6344
+}
+```
+
 ## Development
 
 ```bash