docs: updated readme and changelog about hidden_private_data env var

Author: Yuri Zmytrakov

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Added
 
+- Added environment variable HIDE_PRIVATE_DATA to control visibility of private technical data fields in SFEOS responses. [#518](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/518)
+
 ### Changed
 
 ### Fixed

README.md

@@ -342,6 +342,7 @@ You can customize additional settings in your `.env` file:
 | `STAC_INDEX_ASSETS` | Controls if Assets are indexed when added to Elasticsearch/Opensearch. This allows asset fields to be included in search queries. | `false` | Optional |
 | `USE_DATETIME` | Configures the datetime search behavior in SFEOS. When enabled, searches both datetime field and falls back to start_datetime/end_datetime range for items with null datetime. When disabled, searches only by start_datetime/end_datetime range. | `true` | Optional |
 | `EXCLUDED_FROM_QUERYABLES` | Comma-separated list of fully qualified field names to exclude from the queryables endpoint and filtering. Use full paths like `properties.auth:schemes,properties.storage:schemes`. Excluded fields and their nested children will not be exposed in queryables. | None | Optional |
+| `HIDE_PRIVATE_DATA` | Controls whether private or technical data fields are included in responses. When set to `true`, private data is hidden from the output, when `false`, it is returned as part of the response. | `false` | Optional |
 
 > [!NOTE]
 > The variables `ES_HOST`, `ES_PORT`, `ES_USE_SSL`, `ES_VERIFY_CERTS` and `ES_TIMEOUT` apply to both Elasticsearch and OpenSearch backends, so there is no need to rename the key names to `OS_` even if you're using OpenSearch.