Release React v2 RC

[f1ames]

Are you reporting a feature request or a bug?

Task

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We are really close to have functional next major release (support for React v17 and hooks, see #199). In fact, the code is already there and some polishing left only (e.g. docs).

It will be good to release RC version first so we can have a period of beta-tests and then release as stable.

Since RC will not be default latest on NPM, we need a way to inform users/developers that there is RC version ready to test and use. The information about that should be at least put into:

• [x] Readme file (which is also shown on NPM page).
• [x] Changelog file.
• [x] React section in CKEditor 4 docs (it's there https://ckeditor.com/docs/ckeditor4/latest/guide/dev_react_v2.html).
• [x] Issues which were solved in RC (so the people reporting can switch to newer version already) - see 2.0.0 milestone https://github.com/ckeditor/ckeditor4-react/milestone/9.
• [x] Publish package - already there https://www.npmjs.com/package/ckeditor4-react
• [x] ckeditor4-common-integrations update? As mentioned in https://github.com/ckeditor/ckeditor4-integrations-common/pull/18.
• [x] ~Optionally we can have a blogpost describing there is an RC for React v17 with hooks available on NPM.~ (Let's skip it for now and wait for stable).

[f1ames]

All tasks covered done, closing then :+1: