Add CHANGELOG.md

Author: lorensr

CHANGELOG.md

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+# Changelog
+All notable changes will be documented in this file.
+
+This file applies to the app that is created in Chapter 7 of the book—so the numbered step branches, not `master` ([more info](https://github.com/GraphQLGuide/guide-vue/blob/master/README.md)).
+
+Releases are done with tags in the form `[step]_[version]`, so for instance step `2` of version `1.0.0` is `2_1.0.0`, located here: 
+
+[/GraphQLGuide/guide/releases/tag/2_1.0.0](https://github.com/GraphQLGuide/guide/releases/tag/2_1.0.0)
+
+<!-- ## [Unreleased]
+### Added
+### Removed
+### Changed
+### Fixed -->
+
+## [1.0.0] - 2021-03-01
+
+Released with [`r6`](https://github.com/GraphQLGuide/book/releases/tag/r6)
+
+### Added
+
+- Branches `0–3`