This pull request fixes a documentation error and proposes two tweaks to the documentation.

The top-level crate documentation is fixed to properly use //! instead of ///, meaning that it will properly appear on the home page of the html documentation. (Resolves #4.)

The documentation for the COLORS array has been modified to include array indices in the list of colours, so that a user doesn't have to count the dot points to determine, for example, that white is index 7.

The documentation for the save_png and save_svg functions are expanded to include reasons why a user might choose to use one over the other.