Updated REX-Ray plugin platform support.
REX-Ray added Google Compute Engine support:
@@ -68,7 +68,7 @@ The following plugins exist:
* The [REX-Ray plugin](https://github.com/emccode/rexraycli) is a volume plugin
which is written in Go and provides advanced storage functionality for many
- platforms including EC2, OpenStack, XtremIO, and ScaleIO.
+ platforms including EC2, Google Compute Engine, OpenStack, XtremIO, and ScaleIO.
* The [Contiv Volume Plugin](https://github.com/contiv/volplugin) is an open
source volume plugin that provides multi-tenant, persistent, distributed storage
