Add CHANGELOG and migration notes

CHANGELOG.md

@@ -0,0 +1,58 @@
+# CHANGELOG
+
+## 0.5.0 (Unreleased)
+
+### Breaking Changes
+
+- The [`Cors`](https://lawliet89.github.io/rocket_cors/rocket_cors/struct.Cors.html) struct can no
+    longer be constructed. Instead, you will now construct the options for Cors directly or through
+    deserialization using the
+    [`CorsOptions`](https://lawliet89.github.io/rocket_cors/rocket_cors/struct.CorsOptions.html)
+    struct. Then, you can construct `Cors` for use in Fairings or manual responses using the
+    [`CorsOptions::to_cors`](https://lawliet89.github.io/rocket_cors/rocket_cors/struct.CorsOptions.html#method.to_cors)
+    method.
+- The
+    [`AllowedOrigins`](https://lawliet89.github.io/rocket_cors/rocket_cors/type.AllowedOrigins.html)
+    type has been modified. It is now a typedef of `AllOrSome<Origins>` where
+    [`Origins`](https://lawliet89.github.io/rocket_cors/rocket_cors/struct.Origins.html) is now
+    a struct supporting exact matches or regex matches.
+
+### Migrating existing Code
+
+- Existing use of
+    [`AllowedOrigins::some`](https://docs.rs/rocket_cors/0.4.0/rocket_cors/type.AllowedOrigins.html#method.some)
+    to create exact matches can be replaced simply with
+    [`AllowedOrigins::some_exact`](https://lawliet89.github.io/rocket_cors/rocket_cors/type.AllowedOrigins.html#method.some_exact)
+    instead.
+- Replace all construction of `Cors` struct with `CorsOptions` instead. Then, you can create the
+    `Cors` struct for use in Fairings using the
+    [`CorsOptions::to_cors`](https://lawliet89.github.io/rocket_cors/rocket_cors/struct.CorsOptions.html#method.to_cors)
+    method
+
+    ```diff
+    -fn main() {
+    +fn main() -> Result<(), Error> {
+        let (allowed_origins, failed_origins) = AllowedOrigins::some(&["https://www.acme.com"]);
+        assert!(failed_origins.is_empty());
+
+        // You can also deserialize this
+    -    let options = rocket_cors::Cors {
+    +    let cors = rocket_cors::CorsOptions {
+            allowed_origins: allowed_origins,
+            allowed_methods: vec![Method::Get].into_iter().map(From::from).collect(),
+            allowed_headers: AllowedHeaders::some(&["Authorization", "Accept"]),
+            allow_credentials: true,
+            ..Default::default()
+    -    };
+    +    }
+    +    .to_cors()?;
+
+        rocket::ignite()
+            .mount("/", routes![cors])
+    -        .attach(options)
+    +        .attach(cors)
+            .launch();
+    +
+    +    Ok(())
+    }
+    ```