style guide doesn't provide a reason for prefering interface for object typing

[style guide ](https://github.com/basarat/typescript-book/blob/master/docs/styleguide/styleguide.md) recommends using interface notation for objects, when it extends or implements but unlike all other recommendations doesn't provide reasoning. That is actually one I'd love to know 🙂.

Open docs/styleguide/styleguide.md and locate the recommendation to use interface notation for object typing, especially the section covering extends or implements. Review the surrounding guidance and TypeScript documentation before adding a concise reason for the recommendation; the issue is done when that rationale appears alongside the existing advice.

Written by the indexing model from the issue text.