Add a change log

Closes #2

CHANGELOG.md

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+# Changelog
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
+and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+## [2.0.0] - 2018-01-17
+### Added
+- Attachment actions (buttons) with confirmation support
+- Optional footer attachment support
+- PHP strict typing
+- Options resolver usage for client options
+
+### Changed
+- Moved from Guzzle to PHP-HTTP
+- Client::preparePayload is now private
+- All classes are now final
+- Nexylan namespace. It is now `Nexy\Slack`
+
+### Fixed
+- Bug where message wouldn't get returned on send
+- URL for "Send an attachment" preview
+
+### Removed
+- PHP <7.1 support
+- Array argument for any message and attachment object. Use chained calls instead.
+- Possibility to change the endpoint. Use new instance instead.