Improve README.md

A more friendly presentation of the project =)

### Major changes
- Do not mix a warning with a section about the project
- `isChild: true` - Add code span
- Avoid repetition of "this"
- Improve link to JS: The Right Way

### Other suggestions
Remove "if you want to help".
> "We are working with the community to create this guide, so feel free to contribute!"

Remove "I think".
> "[..] We need a place to put all the best practices and references together, then we can share (good) information and help keep the web more organized."