contributing: Split into different section
contributing: Split into different section

This commit re-organize the text adding sections like:

* How to submit a PR ?
* How to integrate a PR ?
* Are PR tested ?

jbeezley/girder,manthey/girder,opadron/girder,RafaelPalomar/girder,sutartmelson/girder,kotfic/girder,manthey/girder,essamjoubori/girder,salamb/girder,Kitware/girder,msmolens/girder,Xarthisius/girder,data-exp-lab/girder,jbeezley/girder,data-exp-lab/girder,jbeezley/girder,girder/girder,sutartmelson/girder,adsorensen/girder,adsorensen/girder,opadron/girder,kotfic/girder,data-exp-lab/girder,RafaelPalomar/girder,data-exp-lab/girder,manthey/girder,jbeezley/girder,Xarthisius/girder,msmolens/girder,essamjoubori/girder,kotfic/girder,girder/girder,salamb/girder,RafaelPalomar/girder,Xarthisius/girder,essamjoubori/girder,Kitware/girder,salamb/girder,adsorensen/girder,RafaelPalomar/girder,kotfic/girder,girder/girder,opadron/girder,essamjoubori/girder,kotfic/girder,sutartmelson/girder,RafaelPalomar/girder,adsorensen/girder,salamb/girder,essamjoubori/girder,opadron/girder,msmolens/girder,salamb/girder,sutartmelson/girder,Kitware/girder,girder/girder,msmolens/girder,Kitware/girder,adsorensen/girder,msmolens/girder,Xarthisius/girder,opadron/girder,manthey/girder,data-exp-lab/girder,sutartmelson/girder,Xarthisius/girder