docs(readme): document sparse fieldset support

Author: jaredcnance

README.md

@@ -28,6 +28,7 @@ JsonApiDotnetCore provides a framework for building [json:api](http://jsonapi.or
     - [Meta](#meta)
     - [Client Generated Ids](#client-generated-ids)
     - [Custom Errors](#custom-errors)
+    - [Sparse Fieldsets](#sparse-fieldsets)
 - [Tests](#tests)
 
 ## Comprehensive Demo
@@ -397,6 +398,23 @@ public override async Task<IActionResult> PostAsync([FromBody] MyEntity entity)
 }
 ```
 
+### Sparse Fieldsets
+
+We currently support top-level field selection. 
+What this means is you can restrict which fields are returned by a query using the `fields` query parameter, but this does not yet apply to included relationships.
+
+- Currently valid:
+```http
+GET /articles?fields[articles]=title,body HTTP/1.1
+Accept: application/vnd.api+json
+```
+
+- Not yet supported:
+```http
+GET /articles?include=author&fields[articles]=title,body&fields[people]=name HTTP/1.1
+Accept: application/vnd.api+json
+```
+
 ## Tests
 
 I am using DotNetCoreDocs to generate sample requests and documentation.