Add overview, installation, and usage info to README.

Author: cliffano

README.md

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+<img align="right" src="https://raw.github.com/cliffano/packer-post-processor-json-updater/master/avatar.jpg" alt="Avatar"/>
+
+[![Build Status](https://secure.travis-ci.org/cliffano/packer-post-processor-json-updater.png?branch=master)](http://travis-ci.org/cliffano/packer-post-processor-json-updater)
+
+Packer Post-Processor JSON Updater
+----------------------------------
+
+A Packer Post-Processor plugin for updating JSON files.
+
+This is handy when you want to update one or more JSON files after Packer already finish executing the builders.
+
+An example scenario is when you have several Packer templates where the built image of a template becomes the source of a number of other templates.
+
+Installation
+------------
+
+Build `packer-post-processor-json-updater` using `go build` or other build tools, then follow [Packer documentation on installing plugins](https://www.packer.io/docs/extend/plugins.html).
+
+Usage
+-----
+
+Add `json-updater` type to Packer template's post-processor section: 
+
+    "post-processors": [
+        {
+            "type": "json-updater",
+            "ami_id": {
+                "templates/child_template_1.json": [
+                    "variables.aws_source_ami"
+                ],
+                "templates/child_template_2.json": [
+                    "variables.aws_source_ami"
+                ]
+            }
+        }
+    ]
+
+The above `ami_id` configuration indicates that the ID of the newly created AWS AMI will be set as the value of `variables.aws_source_ami` key in `templates/child_template_1.json` and `templates/child_template_2.json` files.
+
+    "variables": {
+        "aws_source_ami": "<ami_id>"
+    }