Create a feature comparison chart with other popular ISO 8601 parsers

[movermeyer]

Potential users should be able to compare the capabilities of ciso8601 with other popular ISO 8601 parsers.

Other parsers might look attractive at first glance, but many have bugs or limitations.

ciso8601 itself has some limitations (I think we already do a good job of documenting that though).

Other libraries have limited support for platforms (ex. udatetime doesn't support Windows).

Make a comparison document or chart that will help people see the differences between the libraries.