Update Post-Processor Docs

Update Post-Processor docs to clarify sequences in packer templates.

website/source/docs/templates/post-processors.html.markdown

@@ -82,6 +82,8 @@ sequence definition. Sequence definitions are used to chain together multiple
 post-processors. An example is shown below, where the artifact of a build is
 compressed then uploaded, but the compressed result is not kept.
 
+It is very important that any post processors that need to be ran in order, be sequenced!
+
 ``` {.javascript}
 {
   "post-processors": [
@@ -96,6 +98,27 @@ compressed then uploaded, but the compressed result is not kept.
 As you may be able to imagine, the **simple** and **detailed** definitions are
 simply shortcuts for a **sequence** definition of only one element.
 
+## Creating Vagrant Boxes in Atlas
+
+It is important to sequence post processors when creating and uploading vagrant boxes to Atlas via Packer. Using a sequence will ensure that the post processors are ran in order and creates the vagrant box prior to uploading the box to Atlas.
+
+``` {.javascript}
+{
+  "post-processors": [
+    [
+      {
+        "type": "vagrant",
+        "keep_input_artifact": false
+      },
+      {
+        "type": "atlas",
+        ...
+      }
+    ]
+  ]
+}
+```
+
 ## Input Artifacts
 
 When using post-processors, the input artifact (coming from a builder or another