Add "Start Here" ribbon

This has been adapted from

   https://gist.github.com/jnothman/0729018fc39b2c30f082#file-forkme-svg

Inkscape tutorial explaining how to clip objects:

   http://inkscapetutorials.wordpress.com/2014/04/22/inkscape-faq-how-do-i-crop-in-inkscape/#clipping
Switch "Motivation" and "What is CMake" section
Tweak position of Contributing step
Add PDF generated based on SVG obtain from 1a39b98
Remove shadow on "Start here" ribbon
Fix acknowledgments alignment + use courrier fond for reference number
Add circle around number for better readability

Couldn't get the "combinumerals" font to work.

See http://www.fontspace.com/the-fontsite/combinumerals
Add PDF generated based on SVG obtain from 1eb6c52