docs(readme): document custom error usage

Author: jaredcnance

README.md

@@ -27,6 +27,7 @@ JsonApiDotnetCore provides a framework for building [json:api](http://jsonapi.or
 	- [Sorting](#sorting)
     - [Meta](#meta)
     - [Client Generated Ids](#client-generated-ids)
+    - [Custom Errors](#custom-errors)
 - [Tests](#tests)
 
 ## Comprehensive Demo
@@ -364,6 +365,38 @@ services.AddJsonApi<AppDbContext>(opt =>
 });
 ```
 
+### Custom Errors
+
+By default, errors will only contain the properties defined by the internal [Error](https://github.com/Research-Institute/json-api-dotnet-core/blob/master/src/JsonApiDotNetCore/Internal/Error.cs) class. However, you can create your own by inheriting from `Error` and either throwing it in a `JsonApiException` or returning the error from your controller.
+
+```chsarp
+// custom error definition
+public class CustomError : Error {
+    public CustomError(string status, string title, string detail, string myProp)
+    : base(status, title, detail)
+    {
+        MyCustomProperty = myProp;
+    }
+    public string MyCustomProperty { get; set; }
+}
+
+// throwing a custom error
+public void MyMethod() {
+    var error = new CustomError("507", "title", "detail", "custom");
+    throw new JsonApiException(error);
+}
+
+// returning from controller
+[HttpPost]
+public override async Task<IActionResult> PostAsync([FromBody] MyEntity entity)
+{
+    if(_db.IsFull)
+        return new ObjectResult(new CustomError("507", "Database is full.", "Theres no more room.", "Sorry."));
+
+    // ...
+}
+```
+
 ## Tests
 
 I am using DotNetCoreDocs to generate sample requests and documentation.