Add example for threshoding

This example was removed in a previous commit.

doc/examples/plot_otsu.py

@@ -0,0 +1,23 @@
+"""
+============
+Thresholding
+============
+
+Thresholding is used to create a binary image. This example uses Otsu's method to calculate the threshold value.
+
+"""
+
+import matplotlib.pyplot as plt
+
+from skimage.data import camera
+from skimage.filter import threshold_otsu
+
+
+image = camera()
+thresh = threshold_otsu(image)
+binary = image > thresh
+
+plt.imshow(binary)
+plt.axis('off')
+plt.show()
+