README: Remove comment about decodeIfPresent

It's a bit self-explanatory, given that the following line is using `try?`, and clutters up the code sample.

Add support for ISO-8601-formatted dates (#8)

Add support for ISO-8601-formatted dates

This change adds support for the common `ISO-8601` date format, by making
it possible to pass a `ISO8601DateFormatter` when encoding or decoding a
date (on supported platforms).

Add Podspec and Swift version file