Update CHANGELOG for v1.0.0

***NO_CI***

Author: nfbot

CHANGELOG.md

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+# Changelog
+
+## [v1.0.0-preview.20](https://github.com/nanoframework/nanoFramework.System.Net.WebSockets/tree/v1.0.0-preview.20) (2021-04-06)
+
+[Full Changelog](https://github.com/nanoframework/nanoFramework.System.Net.WebSockets/compare/7b0814ff45a2c381c0c23964b0c702916529ab5f...v1.0.0-preview.20)
+
+**Documentation and other chores:**
+
+- Udpate NuGets [\#2](https://github.com/nanoframework/nanoframework.System.Net.WebSockets/pull/2)
+
+
+
+\* *This Changelog was automatically generated by [github_changelog_generator](https://github.com/github-changelog-generator/github-changelog-generator)*