Added table of contents to readme

[DougBeney]

First of all, thank you for creating this!! Spent half a day tweaking around with my own string concatenation functions only to find yours to work....much better!

In this pull request I added a table of contents to the readme to make it easier to navigate. It was generated with markdown-toc, an Emacs plugin, but there are a variety of similar plugins for different IDEs and editors to maintain this with. You can also maintain to manually without much of a problem.

You can preview my change here