Add 16.8.0 changelog and update some READMEs (facebook#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 facebook#14119 to changelog

* Add facebook#14744 to changelog

* Fix PR links

* act() method was added to test utils, too

* Updated release date to February 6th