Update README.md
Formatting, links, and citations

* Moved all links to in-text citation in the headings
* Added a reference list at the end with all links
* Added citation for Roy Osherove on string calculator since it appears pretty similar
* Changed the formatting of the string calculator to a bulleted list.
* Updated the intro text
Merge pull request #5 from SeanKilleen/master

Formatting, links, and citations