Add README

Author: rakyll

README.md

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+# OpenCensus Go Stackdriver
+
+[![Build Status](https://travis-ci.org/census-ecosystem/opencensus-go-exporter-stackdriver.svg?branch=master)](https://travis-ci.org/census-ecosystem/opencensus-go-exporter-stackdriver) [![GoDoc][godoc-image]][godoc-url]
+
+Provides OpenCensus exporter support for Stackdriver Monitoring and Stackdriver Trace.
+
+## Installation
+
+```
+$ go get -u contrib.go.opencensus.io/exporter/stackdriver
+```
+
+[godoc-image]: https://godoc.org/contrib.go.opencensus.io/exporter/stackdriver?status.svg
+[godoc-url]: https://godoc.org/contrib.go.opencensus.io/exporter/stackdriver