message: Add link to Evergreen waterfall status
diff: -# MongoDB C++ Driver [![Build Status](https://travis-ci.org/mongodb/mongo-cxx-driver.svg?branch=master)](https://travis-ci.org/mongodb/mongo-cxx-driver)
  +# MongoDB C++ Driver [![Evergreen Build Status](https://evergreen.mongodb.com/static/img/favicon.ico)](https://evergreen.mongodb.com/waterfall/cxx-driver)[![Travis Build Status](https://travis-ci.org/mongodb/mongo-cxx-driver.svg?branch=master)](https://travis-ci.org/mongodb/mongo-cxx-driver)
  Welcome to the MongoDB C++ Driver!
  