Add a "curry an existing function" section

Add to here : https://github.com/basarat/typescript-book/blob/master/docs/tips/currying.md

Stuff thats mentioned here : https://github.com/Microsoft/TypeScript-Handbook/issues/326#issuecomment-228797952

Open docs/tips/currying.md and review the material linked from TypeScript-Handbook issue #326, especially the referenced comment. Add a focused section covering how to curry an existing function, with scope guided by that reference. Done means the section is present in the currying page and clearly explains the requested technique.

Written by the indexing model from the issue text.