add doc for the new Problem type

Routing-error-handlers.md

@@ -2,7 +2,7 @@
 
 You can define your own handlers when a specific http error code occurs.
 
-> Error codes are the http status codes that are bigger than or equal to 400, like 404 not found and 500 internal server.
+Error codes are the http status codes that are bigger than or equal to 400, like 404 not found and 500 internal server error.
 
 Example code:
 
@@ -37,3 +37,29 @@ func index(ctx iris.Context) {
     ctx.View("index.html")
 }
 ```
+
+> Learn more about [[View]].
+
+## The Problem type
+
+Iris has a builtin support for the [Problem Details for HTTP APIs](https://tools.ietf.org/html/rfc7807).
+
+The `Context.Problem` sends a response like JSON but with indent of "  " and
+content type of "application/problem+json".
+
+```go
+func newProductProblem(productName, detail string) iris.Problem {
+    return iris.NewProblem().
+        Type("/product-error").
+        Title("Product validation problem").
+        Detail(detail).
+        Status(iris.StatusBadRequest).
+        Key("productName", productName)
+}
+
+func fireProblem(ctx iris.Context) {
+    ctx.Problem(newProductProblem("product name", "problem error details"))
+}
+```
+
+Full example can be found at [_examples/routing/http-errors](https://github.com/kataras/iris/blob/master/_examples/routing/http-errors/main.go).