website: document config templates

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

@@ -82,7 +82,8 @@ Here is a basic example. It is completely valid except for the access keys:
 ## AMI Name Variables
 
 The AMI name specified by the `ami_name` configuration variable is actually
-treated as a template. Packer provides a set of variables that it will replace
+treated as a [configuration template](/docs/templates/configuration-templates.html).
+Packer provides a set of variables that it will replace
 within the AMI name. This helps ensure the AMI name is unique, as AWS requires.
 
 The available variables are shown below:

website/source/docs/templates/configuration-templates.html.markdown

@@ -0,0 +1,49 @@
+---
+layout: "docs"
+---
+
+# Configuration Templates
+
+Certain configuration parameters within templates are themselves a
+type of "template." These are not Packer templates, but text templates,
+where variables can be used to modify the value of a configuration parameter
+during runtime.
+
+For example, the `ami_name` configuration for the [AMI builder](/docs/builders/amazon-ebs.html)
+is a template. An example value may be `My Packer AMI {{.CreateTime}}`. At
+runtime, this will be turned into `My Packer AMI 1370900368`, where the
+"CreateTime" variable was replaced with the Unix timestamp of when the
+AMI was actually created.
+
+This sort of templating is pervasive throughout Packer. Instead of documenting
+the templating syntax in each location, it is documented once here so
+you know how to use it.
+
+<div class="alert alert-info">
+<strong>For advanced users:</strong> The templates are actually parsed and executed
+using Go's <a href="http://golang.org/pkg/text/template/">text/template</a>
+package. It therefore supports the complete template syntax.
+</div>
+
+## Syntax
+
+99% of the time all you'll need within configuration templates are variables.
+Variables are accessed by using `{{.VARIABLENAME}}`. The "." prefixing the
+variable name signals that you're accessing a variable on the root
+template object. All template directives go between braces. Here is a piece
+of a VMware VMX template that uses variables:
+
+<pre>
+.encoding = "UTF-8"
+displayName = "{{ .Name }}"
+guestOS = "{{ .GuestOS }}"
+</pre>
+
+In this case, the "Name" and "GuestOS" variables will be replaced, potentially
+resulting in a VMX that looks like this:
+
+<pre>
+.encoding = "UTF-8"
+displayName = "packer"
+guestOS = "otherlinux"
+</pre>

website/source/layouts/docs.erb

@@ -38,6 +38,7 @@
 							<li><a href="/docs/templates/introduction.html">Introduction</a></li>
 							<li><a href="/docs/templates/builders.html">Builders</a></li>
 							<li><a href="/docs/templates/provisioners.html">Provisioners</a></li>
+							<li><a href="/docs/templates/configuration-templates.html">Configuration Templates</a></li>
 
 							<li class="nav-header">Builders</li>
 							<li><a href="/docs/builders/amazon-ebs.html">EC2 (AMI)</a></li>