Added principal stresses

Author: ccaprani

docs/source/images/stress/stress_1.png

@@ -332,6 +332,16 @@ Shear Stress (:math:`\sigma_{zxy}`)
 
 ..  automethod:: sectionproperties.analysis.cross_section.StressPost.plot_vector_zxy
     :noindex:
+    
+Major Principal Stress (:math:`\sigma_{1}`)
+""""""""""""""""""""""""""""""""""""
+..  automethod:: sectionproperties.analysis.cross_section.StressPost.plot_stress_1
+    :noindex:
+
+Major Principal Stress (:math:`\sigma_{2}`)
+""""""""""""""""""""""""""""""""""""
+..  automethod:: sectionproperties.analysis.cross_section.StressPost.plot_stress_1
+    :noindex:
 
 von Mises Stress (:math:`\sigma_{vM}`)
 """""""""""""""""""""""""""""""""""""""