Add to README text about custom HttpClient and link to example.

Author: shimatar0

src/lib.rs

@@ -219,6 +219,12 @@
 //!   "query": "wonder"
 //! }
 //! ```
+//!
+//! ### Using users customized HttpClient <!-- omit in TOC -->
+//!
+//! If you want to change the `HttpClient` you can incorporate using the `Client::new_with_client` method.
+//! To use it, you need to implement the `HttpClient Trait`(`isahc` is used by default).
+//! There are [using-reqwest-example](./examples/cli-app-with-reqwest) of using `reqwest`.
 
 #![warn(clippy::all)]
 #![allow(clippy::needless_doctest_main)]