* [Guide] ES6: Prevent object mutation; fixes and enhancements - Remove phantom image - Remove notes for contributors - Change explanation and hints. - Links to markdown * Changed as requested Changed titles as requested by @randelldawson. Note: I'm just used to [MDN](https://developer.mozilla.org/en-US/docs/MDN/Contribute/Guidelines/Writing_style_guide#Page_titles) and other language styles and though it was clearer. Is it ok now for this guide? * fix: removed repl.it link