Cover required_providers in the main docs. (#643)

docs/index.md

@@ -13,6 +13,15 @@ Use the navigation to the left to read about the available resources.
 ## Example Usage
 
 ```hcl
+terraform {
+ required_providers {
+ digitalocean = {
+ source = "digitalocean/digitalocean"
+ version = "~> 2.0"
+ }
+ }
+}
+
 # Set the variable value in *.tfvars file
 # or using -var="do_token=..." CLI option
 variable "do_token" {}
@@ -28,6 +37,9 @@ resource "digitalocean_droplet" "web" {
 }
 ```
 
+-> **Note for Module Developers** Although provider configurations are shared between modules, each module must
+declare its own [provider requirements](https://www.terraform.io/docs/language/providers/requirements.html). See the [module development documentation](https://www.terraform.io/docs/language/modules/develop/providers.html) for additional information.
+
 ## Argument Reference
 
 The following arguments are supported:
@@ -49,4 +61,4 @@ The following arguments are supported:
  used for DigitalOcean Spaces requests. (It defaults to the value of the
  `SPACES_ENDPOINT_URL` environment variable or `https://{{.Region}}.digitaloceanspaces.com`
  if unset.) The provider will replace `{{.Region}}` (via Go's templating engine) with the slug
- of the applicable Spaces region. 
+ of the applicable Spaces region.