Document `transform` option for Chart constructor

As part of the release for d3.chart version 0.2, the Chart constructor
was modify to special-case an optional `transform` option, copying it
onto the new chart instance [1]. Document this behavior in the
description of the Chart constructor.

[1] Commit fbb389de0b2c5722faa46ae6b2041bb62b57e7d9
Merge pull request #90 from jugglinmike/document-transform-options

Document `transform` option for Chart constructor