add wiki link

Author: John Hutcheson

README.md

@@ -13,6 +13,7 @@
     - [DELETE](#delete)
   - [Handling A Response](#handling-a-response)
   - [Method Chart (\*excluding verbs)](#method-chart-heading)
+  - [Wiki](https://github.com/Jhut89/Mailchimp-API-3.0-PHP/wiki) - See for more in depth documentation
 
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