Update Changelog

Author: andfoy

CHANGELOG.md

@@ -0,0 +1,14 @@
+## Version 1.0.0 (2021/05/18)
+
+### Issues Closed
+
+* [Issue 3](https://github.com/python-lsp/python-lsp-black/issues/3) - Update README and add RELEASE instructions
+* [Issue 2](https://github.com/python-lsp/python-lsp-black/issues/2) - Release v1.0.0
+
+In this release 2 issues were closed.
+
+### Pull Requests Merged
+
+* [PR 1](https://github.com/python-lsp/python-lsp-black/pull/1) - PR: Python LSP server migration, by [@andfoy](https://github.com/andfoy)
+
+In this release 1 pull request was closed.