Add brief documentation for backend data export process (LAION-AI#2068)

Close LAION-AI#2026 

This aims to address a couple of questions/issues we've had around how
to export data. The list at the end of the section can be expanded as
more questions are asked.

Author: olliestanley

backend/README.md

@@ -70,3 +70,28 @@ wget localhost:8080/api/v1/openapi.json -O docs/docs/api/openapi.json
 
 Note: The api docs should be automatically updated by the
 `test-api-contract.yaml` workflow.
+
+## Exporting Data
+
+When you have collected some data in the backend database, you can export it
+using the `export.py` script provided in this directory. This can be run from
+the command line using an Python environment with the same requirements as the
+backend itself. The script connects to the database in the same manner as the
+backend and therefore uses the same environmental variables.
+
+A simple usage of the script, to export all English trees which successfully
+passed the review process, may look like:
+
+```bash
+python export.py --lang en --export-file output.jsonl
+```
+
+There are many options available to filter the data which can be found in the
+help message of the script: `python export.py --help`.
+
+**Why isn't my export working?**
+
+Common issues include (WIP):
+
+- The messages have not passed the review process yet so the trees are not ready
+  for export. This can be solved by including the `--include-spam` flag.

backend/export.py

@@ -336,7 +336,7 @@ def parse_args():
     parser.add_argument(
         "--include-spam",
         action="store_true",
-        help="Export including messages with negative review result.",
+        help="Export including messages with no review or negative review result.",
     )
     parser.add_argument(
         "--spam-only",