Update Readme to display well on Github

When a code span contains backticks, GitHub Flavoured Markdown[1] would not parse it properly unless the surrounding backticks are doubled, or even tripled.  Example:

    Bad :   (1) ` `a` `     (2) ` ``b'' `
    Good:   (1) `` `a` ``   (2) ``` ``b'' '''

 [1]: https://help.github.com/articles/github-flavored-markdown