Add Amazon Product API README

Since the instructions are getting more complicated, this commit adds a
README (with new endpoint instructions) to the Amazon Product API. This
has three benefits:

  * It keeps the root README clean and relevant to most people.
  * It allows us to go into more documentation detail.
  * It facilitates an eventual extraction of the Amazon Product API lib.

It might be worth extracting more from the "Getting Amazon Product
Advertising API Working Locally" section into the lib README, but for
now there's just endpoint information.

README.md

@@ -175,7 +175,10 @@ account or login locally:
 
 By the end of this section, you should be able to search Amazon products and
 add items to wishlists on your local machine (when your logged-in user is an
-admin or site manager).
+admin or site manager). *Note: if you're adding a new API endpoint, read more
+[here][API client README].*
+
+[API client README]: lib/amazon_product_api/README.md
 
 **This step is only required for site managers and admins searching/adding
 Amazon products.** If your issue doesn't involve working with the Amazon

lib/amazon_product_api/README.md

@@ -0,0 +1,68 @@
+# Amazon Product Advertising API Client
+
+This folder contains the wrapper code for the Amazon Product Advertising API.
+For more details on the API, check the [Amazon documentation].
+
+[Amazon documentation]: http://docs.aws.amazon.com/AWSECommerceService/latest/DG/Welcome.html
+
+## Adding an Endpoint
+
+All endpoints should be subclassed from `AmazonProductAPI::Endpoint`. In order
+to add a new endpoint, you'll need to modify the template below in a few ways:
+
+  * Prepend any new attributes to the `#initialize` parameter list. Any
+    validations or processing can be done in `#initialize` as shown. Note:
+    setting `aws_credentials` is **required**!
+
+  * Add endpoint-specific request parameters to `#request_params`. These can
+    be found in the Amazon API documentation.
+
+  * Add any post-processing of the API response in `#process_response`.
+
+  * Update the class name and comments.
+
+### Endpoint Template
+
+```ruby
+require 'amazon_product_api/endpoint'
+
+module AmazonProductAPI
+  # Responsible for building and executing <...>
+  #
+  # <endpoint url>
+  #
+  # Contains all specialization logic for this endpoint including request
+  # parameters, parameter validation, and response parsing.
+  class TemplateEndpoint < Endpoint
+    # Add any parameters you need for the specific endpoint.
+    #
+    # Make sure you set `@aws_credentials`-the query won't work without it!
+    def initialize(aws_credentials)
+      # Attribute validations
+      # raise InvalidQueryError, 'reason' if ...
+
+      # Initialize attributes
+      @aws_credentials = aws_credentials
+    end
+
+    private
+
+    attr_accessor :aws_credentials # any other attrs
+
+    # Add any post-processing of the response hash.
+    def process_response(response_hash)
+      ExampleResponse.new(response_hash).item
+    end
+
+    # Include request parameters unique to this endpoint.
+    def request_params
+      {
+        # 'Operation' => 'ItemLookup',
+        # 'IdType' => 'ASIN',
+        # 'ItemId' => 'the item asin',
+        # ...
+      }
+    end
+  end
+end
+```