website: Initial docs for the new "providers" subcommand

This will be fleshed out later as part of more holistic documentation for
the new provider plugin separation, but this is some minimal documentation
for just this subcommand.

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

@@ -41,6 +41,7 @@ Common commands:
     init               Initialize a new or existing Terraform configuration
     output             Read an output from a state file
     plan               Generate and show an execution plan
+    providers          Prints a tree of the providers used in the configuration
     push               Upload this Terraform module to Terraform Enterprise to run
     refresh            Update local state file against real resources
     show               Inspect Terraform state or plan

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

@@ -0,0 +1,36 @@
+---
+layout: "docs"
+page_title: "Command: providers"
+sidebar_current: "docs-commands-providers"
+description: |-
+  The "providers" sub-command prints information about the providers used
+  in the current configuration.
+---
+
+# Command: show
+
+The `terraform providers` command prints information about the providers
+used in the current configuration.
+
+Provider dependencies are created in several different ways:
+
+* Explicit use of a `provider` block in configuration, optionally including
+  a version constraint.
+
+* Use of any resource belonging to a particular provider in a `resource` or
+  `data` block in configuration.
+
+* Existance of any resource instance belonging to a particular provider in
+  the current _state_. For example, if a particular resource is removed
+  from configuration, it continues to create a dependency on its provider
+  until its instances have been destroyed.
+
+This command gives an overview of all of the current dependencies, as an aid
+to understanding why a particular provider is needed.
+
+## Usage
+
+Usage: `terraform show [config-path]`
+
+Pass an explicit configuration path to override the default of using the
+current working directory.

website/source/layouts/docs.erb

@@ -109,6 +109,10 @@
             <a href="/docs/commands/plan.html">plan</a>
           </li>
 
+          <li<%= sidebar_current("docs-commands-providers") %>>
+            <a href="/docs/commands/providers.html">providers</a>
+          </li>
+
           <li<%= sidebar_current("docs-commands-push") %>>
             <a href="/docs/commands/push.html">push</a>
           </li>