Add Dependabot status badge

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

README.md

@@ -2,6 +2,7 @@
 
 [![Travis-CI](https://travis-ci.org/theupdateframework/tuf.svg?branch=develop)](https://travis-ci.org/theupdateframework/tuf)
 [![Coveralls](https://coveralls.io/repos/theupdateframework/tuf/badge.svg?branch=develop)](https://coveralls.io/r/theupdateframework/tuf?branch=develop)
+![Dependabot Status](https://api.dependabot.com/badges/status?host=github&repo=theupdateframework/tuf)
 [![FOSSA Status](https://app.fossa.com/api/projects/custom%2B162%2Fgithub.com%2Ftheupdateframework%2Ftuf.svg?type=shield)](https://app.fossa.com/projects/custom%2B162%2Fgithub.com%2Ftheupdateframework%2Ftuf?ref=badge_shield)
 [![CII](https://bestpractices.coreinfrastructure.org/projects/1351/badge)](https://bestpractices.coreinfrastructure.org/projects/1351)
 [![PyPI](https://img.shields.io/pypi/v/tuf)](https://pypi.org/project/tuf/)