Add what's new entry.

Author: stefmolin

doc/users/next_whats_new/bar_label_formatting.rst

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+Additional format string options in `~matplotlib.axes.Axes.bar_label`
+---------------------------------------------------------------------
+
+The ``fmt`` argument of `~matplotlib.axes.Axes.bar_label` now accepts
+{}-style format strings:
+
+.. code-block:: python
+
+    import matplotlib.pyplot as plt
+
+    fruit_names = ["Coffee", "Salted Caramel", "Pistachio"]
+    fruit_counts = [4000, 2000, 7000]
+
+    fig, ax = plt.subplots()
+    bar_container = ax.bar(fruit_names, fruit_counts)
+    ax.set(ylabel='pints sold', title='Gelato sales by flavor', ylim=(0, 8000))
+    ax.bar_label(bar_container, fmt='{:,.0f}')
+
+It also accepts callables:
+
+.. code-block:: python
+
+    animal_names = ["Lion", "Gazelle", "Cheetah"]
+    mph_speed = [50, 60, 75]
+
+    fig, ax = plt.subplots()
+    bar_container = ax.bar(animal_names, mph_speed)
+    ax.set(ylabel='speed in MPH', title='Running speeds', ylim=(0, 80))
+    ax.bar_label(
+        bar_container, fmt=lambda x: '{:.1f} km/h'.format(x * 1.61)
+    )