digitalocean: update index docs for do_token var

Updates the docs and clarifies the usage of `do_token` variable.

I was experiencing an issue mentioned here hashicorp#124 and so adding more docs should be helpful.

Author: wojtekmach

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

@@ -17,6 +17,9 @@ Use the navigation to the left to read about the available resources.
 ## Example Usage
 
 ```
+# Set the variable value in *.tfvars file or using -var="do_token=..." CLI option
+variable "do_token" {}
+
 # Configure the DigitalOcean Provider
 provider "digitalocean" {
     token = "${var.do_token}"