Add PBR articles

Added PBR articles written by Rémy Bouquet (@nehon)

src/docs/asciidoc/jme3.adoc

@@ -151,6 +151,10 @@ Exporting OgreXML scenes from Blender to JME3
 *  <<jme3/advanced/jme3_srgbpipeline#,Gamma correction or sRGB pipeline>>
 *  <<jme3/shader_video_tutorials#,Videos: jME3 introduction to shaders video tutorial series>>
 *  link:http://www.youtube.com/watch?v=IuEMUFwdheE[Video: jME3 Material with Alpha Channel]
+*  Article: Physically Based Rendering (PBR)
+**  <<jme3/advanced/pbr_part1#,Physically Based Rendering – Part one>>
+**  <<jme3/advanced/pbr_part2#,Physically Based Rendering – Part two>>
+**  <<jme3/advanced/pbr_part3#,Physically Based Rendering – Part three>>
 
 *Physics Integration*