* Better documentation for setupTests.js when ejecting

When running `npm run eject` before creating `src/setupTests.js`, the resulting `package.json` file, won't contain any entry for it - and this is correct in my opinion, since otherwise Jest will crash - but it's useful to have it documented and avoid pointless waste of time.

* Added additional note about src/setupTests.js

Added another note about src/setupTests.js and `npm run eject` in Testing Components section

* Update README.md

* Update README.md

'more' and 'simpler' together is incorrect.

* add support to set REACT_EDITOR to none

* change README message

* change condition to avoid problems with editor being null

* move condition to avoid extra code

* Update README.md with a note that reminds users to prepend REACT_APP_ to their environment variables. 
* Based on #865, where Dan Abramov suggests the person who filed the issue create a PR with these changes.
* Took the language from [Adding Custom Environment Variables](https://github.com/facebookincubator/create-react-app/tree/master/packages/react-scripts/template#adding-custom-environment-variables)

Replace `provide a lot value` with `provide a lot of value`.

* use safer/more aesthetic syntax

* fix typo

The launch configuration code provided is not compatible with VSCode since version 1.19.
The proposed documentation change is based on the response by a VSCode team member.

References:
https://github.com/facebookincubator/create-react-app/issues/3602#issuecomment-352075571
https://github.com/Microsoft/vscode/issues/40293#issuecomment-352017028

* docs: adding section about debugging tests

* docs: removing node-inspector references

* docs: replacing terminal command with npm script

* Update README.md

"subway" by itself is not a proper noun

* Add link to VS Code troubleshooting guide

* Update README.md

Fixed a small grammatical issue that I noticed while browsing the README today

* Change template manifest start_url to dot to make it react-router friendly

* Revert "Change template manifest start_url to dot to make it react-router friendly"

This reverts commit 02f53e81.

* Add a note about PWA manifest and client-side routing

instead of using `&&` to run multiple commands in `package.json`, guide users to use `npm-run-all`

This resolves #3272.

Fixes #3284.