Add examples/demo_live.livemd

Author: wojtekmach

examples/demo_live.livemd

@@ -0,0 +1,42 @@
+# Demo
+
+```elixir
+Mix.install([
+  {:phoenix_playground, "~> 0.1.0"}
+])
+```
+
+## Section
+
+```elixir
+defmodule DemoLive do
+  use Phoenix.LiveView
+
+  def mount(_params, _session, socket) do
+    {:ok, assign(socket, count: 0)}
+  end
+
+  def render(assigns) do
+    ~H"""
+    <span><%= @count %></span>
+    <button phx-click="inc">+</button>
+    <button phx-click="dec">-</button>
+
+    <style type="text/css">
+      body { padding: 1em; }
+      span { font-family: monospace; }
+    </style>
+    """
+  end
+
+  def handle_event("inc", _params, socket) do
+    {:noreply, assign(socket, count: socket.assigns.count + 1)}
+  end
+
+  def handle_event("dec", _params, socket) do
+    {:noreply, assign(socket, count: socket.assigns.count - 1)}
+  end
+end
+
+PhoenixPlayground.start(live: DemoLive)
+```