Add koan illustrating that a process is referenced by a pid

lib/koans/10_processes.ex

@@ -11,6 +11,10 @@ defmodule Processes do
     assert information[:status] == ___
   end
 
+  koan "Processes are referenced by their process ID (pid)" do
+    assert is_pid(self) == ___
+  end
+
   koan "New processes are spawned functions" do
     pid = spawn(fn -> nil end)
 

test/koans/processes_koans_test.exs

@@ -7,6 +7,7 @@ defmodule ProcessesTests do
       true,
       :running,
       true,
+      true,
       "hola!",
       :how_are_you?,
       {:waited_too_long, "I am impatient"},