* Fix broken link in main README

* Fix incorrect link in user guide README

It seems correct URL is https://developers.google.com/web/fundamentals/web-app-manifest/
Current URL (https://developers.google.com/web/fundamentals/engage-and-retain/web-app-manifest/) is redirecting to there.

* avoid TypeError on when registering service workers without config

Fixes #5299

* same before config.onUpdate

* add react-testing-library documentation/examples

* make react-testing-library a heading

* fix typo

* Adds instructions to create a Bootstrap theme with Sass

* Update README.md

* Update README.md

* Update README.md

* Document importing SVGs as React components

* Update README.md

Replace `src/setupTests.js` with `src/setupProxy.js`.

* Updates to README to reflect Workbox usage.

* Update README.md

* Update README.md

* Add additional troubleshooting for Github Pages

* Update README.md

* changed to Migrate to v2.x

* richer explanation

* Update README.md

This reverts commit 11f1aa09.

Husky had an overhaul of the API, see https://github.com/typicode/husky
Therefore the usage advice here has been adjusted.

follwing -> following