* 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)

Fixes #2793

Adds support for the error overlay to detect and open the file with the error in MacVim.

Relates to #2636.

Add TypeScript to **Popular Alternatives**

* Fix path regex match bug

* Use the escape-string-regexp package to escape regex characters

* Remove redundant character escape from path

* Add removed escape of backslashes

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

Replaced `it's` with `its`

the code got updated from `detect()` to `choosePort` while the comment did not.

This fixes the broken link to removed `href-no-hash.md` file, that is no longer available in the [eslint-plugin-jsx-a11y](https://github.com/evcohen/eslint-plugin-jsx-a11y) plugin (since [6.0.0](https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/a14617528cda23c6c88902ace1d083e4e13ac271/CHANGELOG.md))