String Values Enums

https://github.com/Microsoft/TypeScript/pull/15486#issuecomment-303764673 :rose:

* Add example here : https://github.com/basarat/typescript-book/blob/master/docs/enums.md
* Link to `enums` from here : https://github.com/basarat/typescript-book/blob/master/docs/tips/stringEnums.md

Start by reading docs/enums.md and docs/tips/stringEnums.md, following the linked TypeScript discussion for context. Add a string-enum example to docs/enums.md and link to enums from docs/tips/stringEnums.md; done means both documentation updates are present and the links work.

Written by the indexing model from the issue text.