Add 16.8.0 changelog and update some READMEs (#14692)

* Add 16.8.0 changelog * Mention ESLint plugin * Remove experimental notices from the ESLint plugin README * Update CHANGELOG.md * Add more details for Hooks * fix * Set a date * Update CHANGELOG.md Co-Authored-By: gaearon <dan.abramov@gmail.com> * Update CHANGELOG.md * useReducer in changelog * Add to changelog * Update date * Add #14119 to changelog * Add #14744 to changelog * Fix PR links * act() method was added to test utils, too * Updated release date to February 6th