chore: add changelog

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]
+
+## [1.2.0-rc1] - 2021-06-02
+### Added
+- Support for PagerDuty alert channels integration [#53](https://github.com/checkly/terraform-provider-checkly/issues/53).
+
+### Changed
+N/A
+
+### Removed
+N/A
+
+## [1.1.0] - 2021-05-28
+### Added
+- Support for API high frequency checks [#68](https://github.com/checkly/terraform-provider-checkly/issues/68).
+- Add `setupSnippetId` and `teardownSnippetID` to `check_group` resource [#69](https://github.com/checkly/terraform-provider-checkly/issues/69).
+
+### Changed
+N/A
+
+### Removed
+N/A
+
+## [1.0.0] - 2021-04-09
+### Added
+- Apple Silicon support is now added. The Terraform provider now also has `darwin_arm64` binaries
+
+### Changed
+- [🚨 BREAKING CHANGE] The default behavior of assigning all alert channels to checks and check groups is now removed. You can add alerts to your checks and check groups using the `alert_channel_subscription`
+- Support for go1.16
+
+### Removed
+N/A