[dynamic-source-ami] add some docs

website/source/docs/builders/amazon-ebs.html.md

@@ -58,7 +58,8 @@ builder.
     how to set this.](/docs/builders/amazon.html#specifying-amazon-credentials)
 
 -   `source_ami` (string) - The initial AMI used as a base for the newly
-    created machine.
+    created machine. `source_ami_filter` may be used instead to populate this
+    automatically.
 
 -   `ssh_username` (string) - The username to use in order to communicate over
     SSH to the running machine.
@@ -169,6 +170,31 @@ builder.
 -   `skip_region_validation` (boolean) - Set to true if you want to skip 
     validation of the region configuration option.  Defaults to false.
 
+-   `source_ami_filter` (object) - Filters used to populate the `source_ami` field.
+    Example:
+    ``` {.javascript}
+    "source_ami_filter": {
+        "filters": {
+          "virtualization-type": "hvm",
+          "name": "*ubuntu-xenial-16.04-amd64-server-*",
+          "root-device-type": "ebs"
+        },
+        "owners": ["099720109477"],
+        "most_recent": true
+    }
+    ```
+    This selects the most recent Ubuntu 16.04 HVM EBS AMI from Canonical.
+
+-   `source_ami_filter.filters` (map of strings) - filters used to select a `source_ami`.
+    Any filter described in the docs for [DescribeImages](http://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_DescribeImages.html)
+    is valid.
+
+-   `source_ami_filter.owners` (array of strings) - This scopes the AMIs to certain Amazon account IDs.
+    This is helpful to limit the AMIs to a trusted third party, or to your own account.
+
+-   `source_ami_filter.most_recent` (bool) - Selects the newest created image when true.
+    This is most useful for selecting a daily distro build.
+
 -   `spot_price` (string) - The maximum hourly price to pay for a spot instance
     to create the AMI. Spot instances are a type of instance that EC2 starts
     when the current spot price is less than the maximum price you specify. Spot