* 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

 - babel-preset-react-app@5.0.3
 - create-react-app@2.0.3
 - react-dev-utils@6.0.4
 - react-error-overlay@5.0.4
 - react-scripts@2.0.4

* Add `.mjs` support back to webpack, because webpack was not the problem

* Continue toggling `.mjs` to `javascript/auto` mode

* Be more inline with the specification

* Bump old Node to 6

 - babel-plugin-named-asset-import@0.2.2
 - babel-preset-react-app@5.0.2
 - confusing-browser-globals@1.0.3
 - create-react-app@2.0.2
 - eslint-config-react-app@3.0.3
 - react-app-polyfill@0.1.3
 - react-dev-utils@6.0.3
 - react-error-overlay@5.0.3
 - react-scripts@2.0.3

* Adds instructions to create a Bootstrap theme with Sass

* Update README.md

* Update README.md

* Update README.md

 - babel-plugin-named-asset-import@0.2.1
 - babel-preset-react-app@5.0.1
 - confusing-browser-globals@1.0.2
 - create-react-app@2.0.1
 - eslint-config-react-app@3.0.1
 - react-app-polyfill@0.1.2
 - react-dev-utils@6.0.2
 - react-error-overlay@5.0.2
 - react-scripts@2.0.2

* Adds the PnP plugin for Webpack to find dependencies when working under PnP

* Adds configuration for jest

* Adds an e2e test for when using PnP

* Avoids cra from crashing at the engine check

* Avoids cra from crashing when initializing react-scripts

* Makes the ownPath portable

* Fixes linting

* Bumps to pnp-webpack-plugin@1.1.0, removes symlinks: false

* Adds a --use-pnp option

* Pin version

* Document importing SVGs as React components

* Update README.md

* Format SVG React snapshots as <svg> tag with props

* Keep output simpler

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

* Updates to README to reflect Workbox usage.

* Update README.md

* Update README.md