Added basic README to explain usage

README.md

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+INTRODUCTION
+============
+
+This gem is exceptionally simple but also very handy. Let's say you are working in ActiveRecord and you have loaded a collection of objects in a query. You now want to preload associations for these already retrieved objects. This allows you to do that.
+
+USAGE
+=====
+
+The usage of this is very straightforward:
+
+    @users = User.all
+    @users.preload(:posts)
+
+This will eager load all posts for each user even after the initial find query has taken place. You can also use this to preload associations for a member as well:
+
+    @user = User.first
+    @user.preload(:posts)
+
+Why not just do?
+
+    @user.includes(:posts).all
+
+Good question. In most cases this is the better option. This code is for when that approach is not available.