docs: add milestone to readme

README.md

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 ![GitHub repo size](https://img.shields.io/github/repo-size/jjjermiah/nbia-toolkit)
 [![Docker Pulls](https://img.shields.io/docker/pulls/jjjermiah/nbiatoolkit)](https://hub.docker.com/r/jjjermiah/nbiatoolkit)
+<img alt="GitHub milestone details" src="https://img.shields.io/github/milestones/progress-percent/jjjermiah/nbia-toolkit/1?style=flat-square&label=1.0.0%20Stable%20Release%20Milestone&link=https%3A%2F%2Fgithub.com%2Fjjjermiah%2Fnbia-toolkit%2Fmilestone%2F1"><img alt="GitHub milestone details" src="https://img.shields.io/github/milestones/progress/jjjermiah/nbia-toolkit/1?style=flat-square&label=%20&link=https%3A%2F%2Fgithub.com%2Fjjjermiah%2Fnbia-toolkit%2Fmilestone%2F1">
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 # *NBIA Toolkit*
 `nbiatoolkit` is a python package that provides programmatic access to query and download images from the National Biomedical Imaging Archive (**NBIA**) and The Cancer Imaging Archive (**TCIA**) databases.