Show how to use Client::new_with_client (#119)

* Show how to use Client::new_with_client

* new doc on implementation

.gitignore

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-/target
+target/

docs/implementing-client.md

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+# Implementing the client
+
+Once you have generated your client, you'll notice there are two methods to create a new client; `Client::new()` and `Client::new_with_client()`.
+
+The first method will create a basic client without any headers or customisations that aren't already in your OpenAPI specification file:
+
+```rust
+let client = Client::new("https://foo/bar");
+```
+
+Should you need to set custom headers or other reqwest::ClientBuilder methods, you can use `Client::new_with_client()` as shown below.
+
+```rust
+let mut val = reqwest::header::HeaderValue::from_static("super-secret");
+val.set_sensitive(true);
+let mut headers = reqwest::header::HeaderMap::new();
+headers.insert(reqwest::header::AUTHORIZATION, val);
+// Insert more headers if necessary
+
+let client_builder = reqwest::ClientBuilder::new()
+    // Set custom timeout
+    .connect_timeout(Duration::new(60, 0))
+    // Set custom headers
+    .default_headers(headers)
+    .build()
+    .unwrap();
+
+let client Client::new_with_client("https://foo/bar", client_builder);
+```
+
+For more information on available methods, see the [reqwest](https://docs.rs/reqwest/latest/reqwest/struct.ClientBuilder.html) documentation.