Update CHANGELOG for v1.0.0

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Author: nfbot

CHANGELOG.md

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+# Changelog
+
+## [**Changes available only in 'Preview' NuGet packages:**](https://github.com/nanoframework/lib-System.Device.I2c/tree/HEAD)
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+[Full Changelog](https://github.com/nanoframework/lib-System.Device.I2c/compare/afeac3697d8b4cd3130b230ddb8eb37ebc51361a...HEAD)
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+**Implemented enhancements:**
+
+- Add I2cTransferResult and I2cTransferStatus [\#11](https://github.com/nanoframework/lib-System.Device.I2c/pull/11)
+- Initial commit adding System.Device.I2c managed implementation [\#1](https://github.com/nanoframework/lib-System.Device.I2c/pull/1)
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+
+
+\* *This Changelog was automatically generated by [github_changelog_generator](https://github.com/github-changelog-generator/github-changelog-generator)*