Add Dry Transaction migration guide (#39)

Author: timriley

docsite/source/index.html.md

@@ -9,6 +9,7 @@ sections:
   - extensions
   - design-pattern
   - other-monads
+  - migrating-from-dry-transaction
 ---
 
 dry-operation provides an expressive and flexible way for you to model your app's business operations. It provides a lightweight DSL around [dry-monads](/gems/dry-monads/), which allows you to chain together steps and operations with a focus on the happy path, while elegantly handling failures.