stackdriver: add simple readme

public/app/plugins/datasource/stackdriver/README.md

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+# Stackdriver Datasource -  Native Plugin
+
+Grafana ships with built-in support for Google Stackdriver. You just have to add it as a datasource and you will be ready to build dashboards for your Stackdriver metrics.
+
+Read more about it here:
+
+[http://docs.grafana.org/datasources/stackdriver/](http://docs.grafana.org/datasources/stackdriver/)