Add FAQ on Refs

docs/FAQ/UsingRefs.md

@@ -0,0 +1,41 @@
+### Question
+
+When using Reagent, how do I use React's `refs`?
+
+### Answer
+
+Credit: this entry is entirely based on Paulus Esterhazy's [Reagent Mysteries series](https://presumably.de/reagent-mysteries-part-3-manipulating-the-dom.html)
+
+We'll start with a code fragment, because it is worth a 1000 words:
+
+```cljs
+(defn video-ui []
+  (let [!video (clojure.core/atom nil)]    ;; stores the 
+    (fn [{:keys [src]}]
+      [:div
+       [:div
+        [:video {:src src
+                 :style {:width 400}
+                 :ref (fn [el]
+                        (reset! !video el))}]]
+       [:div
+        [:button {:on-click (fn []
+                              (when-let [video @!video] ;; not nil?
+                                (if (.-paused video)
+                                  (.play video)
+                                  (.pause video))))}
+         "Toogle"]]])))
+```
+
+Notes: 
+   1. That's a Form-2 component. That allows us to retain state outside of the renderer `fn`.
+   2. We capture state in `!video`. The state we capture is a reference to a video component. 
+   2. `!video` is a `clojure.core/atom` and not a `reaagent.core/atom`
+   4. On the `:video` component there's a `:ref` callback function which establishes the state in `!video`
+   5. Thereafter, `@!video` is used with the `:button's` `:on-click` to manipulate the `video`
+   5. For full notes [read Paulus' blog post](https://presumably.de/reagent-mysteries-part-3-manipulating-the-dom.html)
+   6. For more background on callback refs, see [React's documentation](https://facebook.github.io/react/docs/more-about-refs.html)
+
+***
+
+Up:  [FAQ Index](../README.md)