vsphere docs: add warning about possible changes

Since we merged this so that the community could collaborate on
improvements, I thought it would be prudent to inform potential users of
the status of the provider so they know what to expect.

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

@@ -15,6 +15,9 @@ The provider needs to be configured with the proper credentials before it can be
 
 Use the navigation to the left to read about the available resources.
 
+~> **NOTE:** The vSphere Provider currently represents _initial support_ and
+therefore may undergo significant changes as the community improves it.
+
 ## Example Usage
 
 ```