Make references to atom explicit, not referred

After extensive discussion in https://github.com/reagent-project/reagent/issues/161 it appears a number of new people get confused by the use of the referred reagent atom. Changing this page (and other parts of the documentation) to use an aliased namespace directly without using a referred version of atom may decrease the number of confused people. This is largely an experiment to see if it helps. (It can't hurt!) :)

README.md

@@ -85,13 +85,13 @@ assuming we have imported Reagent like this:
 
 ```clj
 (ns example
-  (:require [reagent.core :as reagent :refer [atom]]))
+  (:require [reagent.core :as r]))
 ```
 
 State is handled using Reagent's version of `atom`, like this:
 
 ```clj
-(def click-count (atom 0))
+(def click-count (r/atom 0))
 
 (defn state-ful-with-atom []
   [:div {:on-click #(swap! click-count inc)}
@@ -104,7 +104,7 @@ If you want do some setting up when the component is first created, the componen
 
 ```clj
 (defn timer-component []
-  (let [seconds-elapsed (atom 0)]
+  (let [seconds-elapsed (r/atom 0)]
     (fn []
       (js/setTimeout #(swap! seconds-elapsed inc) 1000)
       [:div
@@ -116,7 +116,7 @@ This way you can avoid using React's lifecycle callbacks like `getInitialState`
 But you can still use them if you want to, either using `reagent.core/create-class` or by attaching meta-data to a component function:
 
 ```clj
-(def my-html (atom ""))
+(def my-html (r/atom ""))
 
 (defn plain-component []
   [:p "My html is " @my-html])