Add support for google_compute_project_metadata_item (hashicorp#176)

* Add support for google_compute_project_metadata_item

This allows terraform users to manage single key/value items within the
project metadata map, rather than the entire map itself.

* Update CHANGELOG.md

* Add details about import

docs/r/compute_project_metadata.html.markdown

@@ -8,7 +8,10 @@ description: |-
 
 # google\_compute\_project\_metadata
 
-Manages metadata common to all instances for a project in GCE.
+Manages metadata common to all instances for a project in GCE. If you
+want to manage only single key/value pairs within the project metadata
+rather than the entire set, then use
+[google_compute_project_metadata_item](compute_project_metadata_item.html).
 
 ## Example Usage
 

docs/r/compute_project_metadata_item.html.markdown

@@ -0,0 +1,48 @@
+---
+layout: "google"
+page_title: "Google: google_compute_project_metadata_item"
+sidebar_current: "docs-google-compute-project-metadata-item"
+description: |-
+  Manages a single key/value pair on common instance metadata
+---
+
+# google\_compute\_project\_metadata\_item
+
+Manages a single key/value pair on metadata common to all instances for
+a project in GCE. Using `google_compute_project_metadata_item` lets you
+manage a single key/value setting in Terraform rather than the entire
+project metadata map.
+
+## Example Usage
+
+```hcl
+resource "google_compute_project_metadata_item" "default" {
+  key = "my_metadata"
+  value = "my_value"
+}
+```
+
+## Argument Reference
+
+The following arguments are supported:
+
+* `key` - (Required) The metadata key to set.
+
+* `value` - (Required) The value to set for the given metadata key.
+
+- - -
+
+* `project` - (Optional) The project in which the resource belongs. If it
+    is not provided, the provider project is used.
+
+## Attributes Reference
+
+Only the arguments listed above are exposed as attributes.
+
+## Import
+
+Project metadata items can be imported using the `key`, e.g.
+
+```
+$ terraform import google_compute_project_metadata_item.default my_metadata
+```

google.erb

@@ -153,6 +153,10 @@
       <a href="/docs/providers/google/r/compute_project_metadata.html">google_compute_project_metadata</a>
       </li>
 
+      <li<%= sidebar_current("docs-google-compute-project-metadata-item") %>>
+      <a href="/docs/providers/google/r/compute_project_metadata_item.html">google_compute_project_metadata_item</a>
+      </li>
+
       <li<%= sidebar_current("docs-google-compute-region-backend-service") %>>
       <a href="/docs/providers/google/r/compute_region_backend_service.html">google_compute_region_backend_service</a>
       </li>