Add CII badge to README
[![Build Status](https://travis-ci.org/spdx/tools.svg?branch=master)](https://travis-ci.org/spdx/tools)
+ [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/734/badge)](https://bestpractices.coreinfrastructure.org/projects/734)
+
# Overview
The Software Package Data Exchange (SPDX) specification is a standard format for communicating the components, licenses and copyrights associated with a software package.
