Add CHANGELOG.md and UPGRADE.md

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](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased](https://github.com/mll-lab/laravel-graphql-playground/compare/v2.0.0...master)
+
+## [v2.0.0](https://github.com/mll-lab/laravel-graphql-playground/compare/v1.1.0...v2.0.0)
+
+### Added
+
+- Support Lumen
+- Load routes through a file: `src/routes.php`
+
+### Changed
+
+- Make the `GraphQLPlaygroundController` use `__invoke()` instead of `get()`
+- Move the `route_name` configuration option into `route.uri`
+
+### Removed
+
+- Disable the `route.domain` configuration option by default
+
+## Pre-v2
+
+We just started maintaining a changelog starting from v2.
+
+If someone wants to make one for previous versions, PR's are welcome.

UPGRADE.md

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+# Upgrade guide
+
+This document provides guidance for upgrading between major versions.
+
+## v1 to v2
+
+Compare your `graphql-playground.php` against the latest [default configuration](src/graphql-playground.php).
+The configuration options for the `route` changed.
+
+If you were calling the `GraphQLPlaygroundController` directly, change the reference
+to use `__invoke()`:
+
+```diff
+-'uses' => \MLL\GraphQLPlayground\GraphQLPlaygroundController::class . '@get',
++'uses' => \MLL\GraphQLPlayground\GraphQLPlaygroundController::class,
+```