Merge branch 'custom_vertex_attrib_doc' into 'master'

documentation about making use of custom vertex attributes

(cherry picked from commit 833a6b80123a830940f7b27682d74f93513a47c4)

See merge request !75
8 jobs for master
Status Name Job ID Coverage
  Test
passed clang-c++11-Qt4 #3876
Linux MultiThreads

00:08:00

passed clang-c++11-Qt5 #3880
Linux MultiThreads

00:09:48

passed clang-c++98-Qt4 #3878
Linux MultiThreads

00:12:26

passed clang-c++98-Qt5 #3882
Linux MultiThreads

00:13:41

passed gcc-c++11-Qt4 #3875
Linux

00:03:30

passed gcc-c++11-Qt5 #3879
Linux

00:04:18

passed gcc-c++98-Qt4 #3877
Linux

00:02:52

passed gcc-c++98-Qt5 #3881
Linux

00:03:34