Add section to readme

Author: jonhealy1

CHANGELOG.md

@@ -11,7 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 - Added `USE_DATETIME` environment variable to configure datetime search behavior in SFEOS. [#452](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/452)
 - GET `/collections` collection search sort extension ex. `/collections?sortby=+id`. [#456](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/456)
-- GET `/collections` collection search fields extension ex. `/collections?fields=id,title`. [#457](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/457)
+- GET `/collections` collection search fields extension ex. `/collections?fields=id,title`. [#465](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/465)
 
 ### Changed
 

README.md

@@ -36,11 +36,10 @@ SFEOS (stac-fastapi-elasticsearch-opensearch) is a high-performance, scalable AP
 - **Scale to millions of geospatial assets** with fast search performance through optimized spatial indexing and query capabilities
 - **Support OGC-compliant filtering** including spatial operations (intersects, contains, etc.) and temporal queries
 - **Perform geospatial aggregations** to analyze data distribution across space and time
+- **Enhanced collection search capabilities** with support for sorting and field selection
 
 This implementation builds on the STAC-FastAPI framework, providing a production-ready solution specifically optimized for Elasticsearch and OpenSearch databases. It's ideal for organizations managing large geospatial data catalogs who need efficient discovery and access capabilities through standardized APIs.
 
-
-
 ## Common Deployment Patterns
 
 stac-fastapi-elasticsearch-opensearch can be deployed in several ways depending on your needs:
@@ -72,6 +71,7 @@ This project is built on the following technologies: STAC, stac-fastapi, FastAPI
   - [Common Deployment Patterns](#common-deployment-patterns)
   - [Technologies](#technologies)
   - [Table of Contents](#table-of-contents)
+  - [Collection Search Extensions](#collection-search-extensions)
   - [Documentation \& Resources](#documentation--resources)
   - [Package Structure](#package-structure)
   - [Examples](#examples)
@@ -113,6 +113,26 @@ This project is built on the following technologies: STAC, stac-fastapi, FastAPI
   - [Gitter Chat](https://app.gitter.im/#/room/#stac-fastapi-elasticsearch_community:gitter.im) - For real-time discussions
   - [GitHub Discussions](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/discussions) - For longer-form questions and answers
 
+## Collection Search Extensions
+
+SFEOS implements extended capabilities for the `/collections` endpoint, allowing for more powerful collection discovery:
+
+- **Sorting**: Sort collections by sortable fields using the `sortby` parameter
+  - Example: `/collections?sortby=+id` (ascending sort by ID)
+  - Example: `/collections?sortby=-id` (descending sort by ID)
+
+- **Field Selection**: Request only specific fields to be returned using the `fields` parameter
+  - Example: `/collections?fields=id,title,description`
+  - This helps reduce payload size when only certain fields are needed
+
+These extensions make it easier to build user interfaces that display and navigate through collections efficiently.
+
+> **Note**: Sorting is only available on fields that are indexed for sorting in Elasticsearch/OpenSearch. With the default mappings, you can sort on:
+> - `id` (keyword field)
+> - `extent.temporal.interval` (date field)
+>
+> Text fields like `title` and `description` are not sortable by default as they use text analysis for better search capabilities. Attempting to sort on these fields will result in a `BadRequestError` with a message about fielddata being disabled.
+
 ## Package Structure
 
 This project is organized into several packages, each with a specific purpose: