docs: Add examples to query filter doc (#843)

src/filters/query.rs

@@ -10,6 +10,58 @@ use crate::reject::{self, Rejection};
 /// Creates a `Filter` that decodes query parameters to the type `T`.
 ///
 /// If cannot decode into a `T`, the request is rejected with a `400 Bad Request`.
+///
+/// # Example
+///
+/// ```
+/// use std::collections::HashMap;
+/// use warp::{
+///     http::Response,
+///     Filter,
+/// };
+///
+/// let route = warp::any()
+///     .and(warp::query::<HashMap<String, String>>())
+///     .map(|map: HashMap<String, String>| {
+///         let mut response: Vec<String> = Vec::new();
+///         for (key, value) in map.into_iter() {
+///             response.push(format!("{}={}", key, value))
+///         }
+///         Response::builder().body(response.join(";"))
+///     });
+/// ```
+///
+/// You can define your custom query object and deserialize with [Serde][Serde]. Ensure to include
+/// the crate in your dependencies before usage.
+///
+/// ```
+/// use serde_derive::{Deserialize, Serialize};
+/// use std::collections::HashMap;
+/// use warp::{
+///     http::Response,
+///     Filter,
+/// };
+///
+/// #[derive(Serialize, Deserialize)]
+/// struct FooQuery {
+///     foo: Option<String>,
+///     bar: u8,
+/// }
+///
+/// let route = warp::any()
+///     .and(warp::query::<FooQuery>())
+///     .map(|q: FooQuery| {
+///         if let Some(foo) = q.foo {
+///             Response::builder().body(format!("foo={}", foo))
+///         } else {
+///             Response::builder().body(format!("bar={}", q.bar))
+///         }
+///     });
+/// ```
+///
+/// For more examples, please take a look at [examples/query_string.rs](https://github.com/seanmonstar/warp/blob/master/examples/query_string.rs).
+///
+/// [Serde]: https://docs.rs/serde
 pub fn query<T: DeserializeOwned + Send + 'static>(
 ) -> impl Filter<Extract = One<T>, Error = Rejection> + Copy {
     filter_fn_one(|route| {