Added a code sample

I have not tested this change, I apologize for that.

doc/source/api/process_lock.rst

@@ -17,3 +17,18 @@ Decorators
 ----------
 
 .. autofunction:: fasteners.process_lock.interprocess_locked
+
+.. code-block:: python
+    # Launch multiple of these at the same time to see the lock in action
+    import time
+    import fasteners
+
+    @fasteners.process_lock.interprocess_locked('tmp_lock_file')
+    def test():
+        for i in range(10):
+            print('I have the lock')
+            time.sleep(1)
+
+
+    print('Waiting for the lock')
+    test()