Add an example for leader election based multiple controller

examples/leader_example/config/config.exs

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+# This file is responsible for configuring your application
+# and its dependencies with the aid of the Mix.Config module.
+use Mix.Config
+
+config :libcluster,
+  topologies: [
+    gossip_example: [
+      strategy: Cluster.Strategy.Gossip,
+      config: [
+        port: 45_892,
+        if_addr: {0, 0, 0, 0},
+        multicast_addr: {230, 1, 1, 251},
+        multicast_ttl: 1]]]
+
+config :logger,
+  level: :debug,
+  format: "$date $time [$level] $metadata$message\n",
+  metadata: [:application],
+  handle_otp_reports: true
+
+import_config "#{Mix.env}.exs"

examples/leader_example/config/dev.exs

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+# This file is responsible for configuring your application
+# and its dependencies with the aid of the Mix.Config module.
+use Mix.Config

examples/leader_example/config/node1.exs

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+use Mix.Config
+
+config :tres,
+  protocol:        :tcp,
+  port:            6653,
+  max_connections: 10,
+  num_acceptors:   10,
+  callback_module: LeaderExample.Leader,
+  callback_args:   []

examples/leader_example/config/node2.exs

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+use Mix.Config
+
+config :tres,
+  protocol:        :tcp,
+  port:            6633,
+  max_connections: 10,
+  num_acceptors:   10,
+  callback_module: LeaderExample.Leader,
+  callback_args:   []