Add what's new entry about fixed kerning.

doc/users/next_whats_new/2019-08-14-ES.rst

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+Kerning adjustments now use correct values
+------------------------------------------
+
+Due to an error in how kerning adjustments were applied, previous versions of
+Matplotlib would under-correct kerning. This version will now correctly apply
+kerning (for fonts supported by FreeType). To restore the old behavior (e.g.,
+for test images), you may set :rc:`text.kerning_factor` to 6 (instead of 0).
+Other values have undefined behavior.
+
+.. plot::
+
+   import matplotlib.pyplot as plt
+
+   # Use old kerning values:
+   plt.rcParams['text.kerning_factor'] = 6
+   fig, ax = plt.subplots()
+   ax.text(0.0, 0.05, 'BRAVO\nAWKWARD\nVAT\nW.Test', fontsize=56)
+   ax.set_title('Before (text.kerning_factor = 6)')
+
+Note how the spacing between characters is uniform between their bounding boxes
+(above). With corrected kerning (below), slanted characters (e.g., AV or VA)
+will be spaced closer together, as well as various other character pairs,
+depending on font support (e.g., T and e, or the period after the W).
+
+.. plot::
+
+   import matplotlib.pyplot as plt
+
+   # Use new kerning values:
+   plt.rcParams['text.kerning_factor'] = 0
+   fig, ax = plt.subplots()
+   ax.text(0.0, 0.05, 'BRAVO\nAWKWARD\nVAT\nW.Test', fontsize=56)
+   ax.set_title('After (text.kerning_factor = 0)')