Add explanation of reactivity to readme (#249)

readme.md

@@ -195,6 +195,39 @@ Once you have tasted this power there is no going back and languages
 without AST based macro system simply don't cut it anymore.
 
 
+## Reactivity
+
+Karax's reactivity model is different to mainstream frameworks, who usually implement it by creating reactive state. Karax instead reacts to events.
+
+This approach is simpler and easier to reason about, with the tradeoff being that events need to be wrapped to trigger a redraw. Karax does this for you with dom event handlers (`onclick`, `keyup`, etc) and ajax network calls (when using `karax/kajax`), but you will need to add it for things outside of that (websocket messages, document timing functions, etc).
+
+`karax/kdom` includes a definition for `setInterval`, the browser api that repeatedly calls a given function. By default it is not reactive, so this is how we might add reactivity with a call to `redraw`:
+
+```nim
+include karax/prelude
+import karax/kdom except setInterval
+
+proc setInterval(cb: proc(), interval: int): Interval {.discardable.} =
+  kdom.setInterval(proc =
+    cb()
+    if not kxi.surpressRedraws: redraw(kxi)
+  , interval)
+
+var v = 10
+
+proc update =
+  v += 10
+
+setInterval(update, 200)
+
+proc main: VNode =
+  buildHtml(tdiv):
+    text $v
+
+setRenderer main
+```
+
+
 ## Attaching data to an event handler