ENH: Add Contributing.md

Initial version of this document has adapted from Girder contribution
process document [1] by @jcfr and was updated during the Slicer weekly
hangouts to incorporate feedback from the community.

It includes the following sections:

• The PR Process, Circle CI, and Related Gotchas
  • How to submit a PR ?
  • How to efficiently contribute ?
  • How to integrate a PR ?
  • Automatic testing of pull requests
  • Nightly tests
  • Decision-making process
  • Benevolent dictators for life

[1] https://github.com/girder/girder/blob/master/CONTRIBUTING.md

[ci skip]

Co-authored-by: Johan Andruejol <johan.andruejol@kitware.com>
Co-authored-by: Andriy Fedorov <fedorov@bwh.harvard.edu>
Co-authored-by: Alexis Girault <alexis.girault@kitware.com>
Co-authored-by: Michael Halle <mhalle@bwh.harvard.edu>
Co-authored-by: Ron Kikinis <kikinis@bwh.harvard.edu>
Co-authored-by: Andras Lasso <lasso@queensu.ca>
Co-authored-by: Matt McCormick <matt.mccormick@kitware.com>
Co-authored-by: Isaiah Norton <inorton@bwh.harvard.edu>
Co-authored-by: Steve Pieper <pieper@isomics.com>
Co-authored-by: Csaba Pinter <csaba.pinter@queensu.ca>
Co-authored-by: Hina Shah <hina.shah@kitware.com>
Co-authored-by: Dzenan Zukic <dzenan.zukic@kitware.com>

Alphabetically ordered by last name.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25838 3bd1e089-480b-0410-8dfb-8563597acbee