docs: Document how to configure nested stream maps values with enviro…

…nment variables in Meltano

docs/stream_maps.md

@@ -778,3 +778,31 @@ the `key_properties` in an extract-load pipeline. For instance, it is common to
 "append-only" loading behavior in certain targets, as may be required for historical reporting. This does not change the
 underlying nature of the `primary_key` configuration in the upstream source data, only how it will be landed or deduped
 in the downstream source.
+
+
+### Q: How do I use Meltano environment variables to configure stream maps?
+
+**Answer:** Environment variables in Meltano can be used to configure stream maps, but you first need to add the corresponding settings
+to your plugins `settings` option. For example:
+
+```yaml
+plugins:
+  extractors:
+  - name: tap-csv
+    variant: meltanolabs
+    pip_url: git+https://github.com/MeltanoLabs/tap-csv.git
+    settings:
+    - name: stream_maps.customers.email
+    - name: stream_maps.customers.email_domain
+    - name: stream_maps.customers.email_hash
+    - name: stream_maps.customers.__else__
+    - name: stream_maps.stream_map_config
+```
+
+Then, you can set the following environment variables:
+
+```shell
+TAP_CSV_STREAM_MAPS_CUSTOMERS_EMAIL_DOMAIN='email.split("@")[-1]'
+TAP_CSV_STREAM_MAPS_CUSTOMERS_EMAIL_HASH='md5(config["hash_seed"] + email)'
+TAP_CSV_STREAM_MAP_CONFIG_HASH_SEED='01AWZh7A6DzGm6iJZZ2T'
+```