Add codecov badge (open-telemetry#77)

Author: mayurkale22

README.md

@@ -1,6 +1,7 @@
 # OpenTelemetry - distributed tracing and stats collection framework
 [![Gitter chat][node-gitter-image]][node-gitter-url]
 [![CircleCI][circleci-image]][circleci-url]
+[![Coverage Status][codecov-image]][codecov-url]
 
 ## Contributing
 We'd love your help!. Use tags [up-for-grabs][up-for-grabs-issues] and
@@ -15,3 +16,5 @@ We have a weekly SIG meeting! See the [community page](https://github.com/open-t
 [node-gitter-url]: https://gitter.im/open-telemetry/opentelemetry-node?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge
 [up-for-grabs-issues]: https://github.com/open-telemetry/OpenTelemetry-js/issues?q=is%3Aissue+is%3Aopen+label%3Aup-for-grabs
 [good-first-issues]: https://github.com/open-telemetry/OpenTelemetry-js/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22
+[codecov-image]: https://codecov.io/gh/open-telemetry/opentelemetry-js/branch/master/graph/badge.svg
+[codecov-url]: https://codecov.io/gh/open-telemetry/opentelemetry-js/branch/master/