add OpenStack to list of IaaS providers

Author: jrperritt

website/source/docs/providers/index.html.markdown

@@ -14,7 +14,7 @@ etc. Almost any infrastructure noun can be represented as a resource in Terrafor
 
 Terraform is agnostic to the underlying platforms by supporting providers. A provider
 is responsible for understanding API interactions and exposing resources. Providers
-generally are an IaaS (e.g. AWS, DigitalOcean, GCE), PaaS (e.g. Heroku, CloudFoundry),
+generally are an IaaS (e.g. AWS, DigitalOcean, GCE, OpenStack), PaaS (e.g. Heroku, CloudFoundry),
 or SaaS services (e.g. Atlas, DNSimple, CloudFlare).
 
 Use the navigation to the left to read about the available providers.