Add license scanning badge
In CNCF, we are starting to experiment with automated license scanning across our projects to ensure compliance to the CNCF IP Policy. For now, we're experimenting with FOSSA:
![banner](/docs/images/containerd-dark.png?raw=true)
[![Build Status](https://travis-ci.org/containerd/containerd.svg?branch=master)](https://travis-ci.org/containerd/containerd)
+[![FOSSA Status](https://app.fossa.io/api/projects/git%2Bhttps%3A%2F%2Fgithub.com%2Fcontainerd%2Fcontainerd.svg?type=shield)](https://app.fossa.io/projects/git%2Bhttps%3A%2F%2Fgithub.com%2Fcontainerd%2Fcontainerd?ref=badge_shield)
containerd is an industry-standard container runtime with an emphasis on simplicity, robustness and portability. It is available as a daemon for Linux and Windows, which can manage the complete container lifecycle of its host system: image transfer and storage, container execution and supervision, low-level storage and network attachments, etc..
