Update CHANGELOG for v1.0.0

***NO_CI***

Author: nfbot

CHANGELOG.md

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+# Changelog
+
+## [**Changes available only in 'Preview' NuGet packages:**](https://github.com/nanoframework/lib-System.Device.Gpio/tree/HEAD)
+
+[Full Changelog](https://github.com/nanoframework/lib-System.Device.Gpio/compare/e316141a3fa09e108056b5532d334cda8608bfbc...HEAD)
+
+**Implemented enhancements:**
+
+- Move to fully native implementation [\#2](https://github.com/nanoframework/lib-System.Device.Gpio/pull/2)
+
+
+
+\* *This Changelog was automatically generated by [github_changelog_generator](https://github.com/github-changelog-generator/github-changelog-generator)*