add further (upstream) documentation for inkscape's transform attribute setting

[dvzrv]

IMHO it would be a good idea to provide further information to users in regards to inkscape's transform attribute settings (and especially its quirks) for SVG objects.
This setting is relevant for the XML attribute modifications type of transformation for objects.

Rationale:
When an SVG file has a group that is supposed to be used for a transformation and this group has its transform attribute set already, this can lead to very unexpected results: e.g. a matrix() transform can be stuck on a group, due to one of its child objects requiring it. When using a translate() function on such a group, its matrix() function will be reset and afterwards the translate() function will be called. As the matrix() function was in reality only applied to one of the child objects, it will be resize/move the group to very unexpected dimensions/locations and then apply the additional transform.

I think it could be worthwhile mentioning this, as I'm fairly sure that users might spend quite some time on figuring this out on their own (I just did). A note could be made reasonably short and e.g. have a link to some stackoverflow issue (such as this one).