docs: provide basic auth example (#1362)

src/async_impl/request.rs

@@ -228,6 +228,19 @@ impl RequestBuilder {
     }
 
     /// Enable HTTP basic authentication.
+    ///
+    /// ```rust
+    /// # use reqwest::Error;
+    ///
+    /// # async fn run() -> Result<(), Error> {
+    /// let client = reqwest::Client::new();
+    /// let resp = client.delete("http://httpbin.org/delete")
+    ///     .basic_auth("admin", Some("good password"))
+    ///     .send()
+    ///     .await?;
+    /// # Ok(())
+    /// # }
+    /// ```
     pub fn basic_auth<U, P>(self, username: U, password: Option<P>) -> RequestBuilder
     where
         U: fmt::Display,