Update readme for opentelemetry-core (open-telemetry#113)

related to open-telemetry#96

Signed-off-by: Olivier Albertini <[email protected]>

packages/opentelemetry-core/README.md

@@ -1,3 +1,16 @@
 # OpenTelemetry Core
 
-TODO
+This package provides default and no-op implementations of the OpenTelemetry types for trace and metrics. It's intended for use both on the server and in the browser.
+
+## Useful links
+- For more information on OpenTelemetry, visit: <https://opentelemetry.io/>
+- For more about OpenTelemetry JavaScript: <https://github.com/open-telemetry/opentelemetry-js>
+- For help or feedback on this project, join us on [gitter][gitter-url]
+
+## License
+
+Apache 2.0 - See [LICENSE][license-url] for more information.
+
+[gitter-image]: https://badges.gitter.im/open-telemetry/opentelemetry-js.svg
+[gitter-url]: https://gitter.im/open-telemetry/opentelemetry-node?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge
+[license-url]: https://github.com/open-telemetry/opentelemetry-js/blob/master/LICENSE