add redis instrumentation plugins by default (#471)

Author: vmarchaud

README.md

@@ -28,6 +28,8 @@ OpenCensus can collect tracing data automatically using plugins. Users can also
 - [HTTPS](https://github.com/census-instrumentation/opencensus-node/blob/master/packages/opencensus-instrumentation-https/README.md)
 - [HTTP2](https://github.com/census-instrumentation/opencensus-node/blob/master/packages/opencensus-instrumentation-http2/README.md)
 - [GRPC](https://github.com/census-instrumentation/opencensus-node/blob/master/packages/opencensus-instrumentation-grpc/README.md)
+- [IORedis](https://github.com/census-instrumentation/opencensus-node/blob/master/packages/opencensus-instrumentation-ioredis/README.md)
+- [Redis](https://github.com/census-instrumentation/opencensus-node/blob/master/packages/opencensus-instrumentation-redis/README.md)
 
 ## Propagation