Merge branch 'preparing-release' into 'master'

doc/plot/plot_Znorm.py

doc/plot/plot_align_faces.py

doc/plot/plot_iVector.py

doc/plot/plot_kmeans.py

doc/plot/plot_landmarks.py

doc/plot/plot_multiple_faces.py

doc/plot/plot_single_faces.py

doc/plot/show_lena.py

doc/plot/similarity.py

doc/plot/single_face_lmks.py

doc/plot/sobel.py

doc/plot/transform.py

doc/plot/wavelet.py

doc/pyplots/detect_skin_pixels.py

doc/readme_index.rst

packages.txt

requirements.txt