meta: add CHANGELOG.md

Author: Fishrock123

CHANGELOG.md

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+# Changelog
+
+All notable changes to surf will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://book.async.rs/overview/stability-guarantees.html).
+
+## [Unreleased]
+
+## [4.0.0] - 2020-07-09
+
+This release allows `HttpClient` to be used as a dynamic Trait object.
+
+- `HttpClient`: removed `Clone` bounds.
+- `HttpClient`: removed `Error` type.
+
+## [3.0.0] - 2020-05-29
+
+This patch updates `http-client` to `http-types 2.0.0` and a new version of `async-h1`.
+
+## Changes
+- http types and async-h1 for 2.0.0 #27
+
+## [2.0.0] - 2020-04-17
+
+## Added
+- Added a new backend: `h1-client` https://github.com/http-rs/http-client/pull/22
+
+## Changed
+- All types are now based from `hyperium/http` to `http-types` https://github.com/http-rs/http-client/pull/22