Update Readme with two datasets named LINZ and UGRC

[xiaofang007]

Hi Robin, I create a pull request to contribute two (bbox annotated datasets) that are part of the contribution of this ICCV paper on aerial imagery (http://arxiv.org/abs/2507.20976) to this repo. The two datasets are collected from New Zealand and Utah, which are used for small vehicle (mostly car) detection in aerial imagery. Thanks.