Update documentation about plugin features

[jaspermdegroot]

Now that WordPress 4.4 has been released I think it's good if we update the documentation in our readme files.
We should explain that the plugin still can be used to include Picturefill and for the advanced image compression. People should also keep using it if you started to use the plugin before version 2.5, because we need to replace data-sizes when the srcset is already in the markup.
You shouldn't be using the plugin to make images responsive because that means that you are using deprecated functions and/or didn't update to WordPress 4.4 yet.

[tevko]

👍

[jaspermdegroot]

Fixed in 768cec8