Add deprecation warnings

Author: samudary

README.md

@@ -122,6 +122,8 @@ as methods on the client object. The following methods are currently available:
 
 #### Orders
 
+**Note:** The beta purchases endpoint has been deprecated and its methods have been removed from the gem except `create_purchase`, which now sends requests to the Order creation endpoint [here](https://developer.drip.com/#orders)
+
 | Actions                    | Methods                                              |
 | :------------------------- | :--------------------------------------------------- |
 | Create or update an order  | `#create_or_update_order(email, options = {})`       |

lib/drip/client/purchases.rb

@@ -9,7 +9,12 @@ module Purchases
       #                      Drip API docs for the required schema.
       # Returns a Drip::Response.
       # See https://developer.drip.com/#orders
+      #
+      # DEPRECATED: The beta Purchase endpoint has been deprecated and this method now sends
+      # requests to the Order creation endpoint. Please use `create_or_update_order` instead
       def create_purchase(email, amount, options = {})
+        warn "[DEPRECATED] `create_purchase` is deprecated. Please use `create_or_update_order` instead."
+
         data = options.merge({ amount: amount, email: email })
         post "#{account_id}/orders", generate_resource("orders", data)
       end