README: add ci status badge for master

README.md

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 # SecureDrop Debian Packaging
 
+[![CircleCI](https://circleci.com/gh/freedomofpress/securedrop-debian-packaging/tree/master.svg?style=svg)](https://circleci.com/gh/freedomofpress/securedrop-debian-packaging/tree/master)
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 This repository contains the packaging files and tooling for building Debian packages for projects for the alpha [SecureDrop Workstation](https://github.com/freedomofpress/securedrop-workstation) based on Qubes OS. Packages are placed on `apt-test-qubes.freedom.press` for installation in Debian-based TemplateVMs. These packages are not yet ready for use in a production environment.
 
 ## Packaging a Python-based SecureDrop project