Add example for LR scheduler

A short and simple example, especially for new users. It helps for a general understanding on how to use this callback. Open for feedback and suggestions. ;)

tensorflow/python/keras/callbacks.py

@@ -1168,6 +1168,23 @@ class LearningRateScheduler(Callback):
           (integer, indexed from 0) and returns a new
           learning rate as output (float).
       verbose: int. 0: quiet, 1: update messages.
+  
+  ```python
+  # Firstly, let's create a function which 
+  # maps a given epoch to a learning rate.
+  # This function would keep the learning rate 
+  # constant at 0.001 for the first ten epochs and  
+  # let it decrease exponentially after that.
+  def scheduler(epoch):
+    if epoch < 10:
+      return 0.001
+    else:
+      return 0.001 * tf.math.exp(0.1 * (10 - epoch))
+  # Next, we need to set up the callback and train the model.
+  callback = tf.keras.callbacks.LearningRateScheduler(scheduler)
+  model.fit(data, labels, epochs=100, callbacks=[callback], 
+            validation_data=(val_data, val_labels))
+  ```
   """
 
   def __init__(self, schedule, verbose=0):