Update contributing guide

Following the discussions in #562, #565, and [pcl-dev](http://www.pcl-developers.org/Copyrights-in-license-td5709405.html), this pull request makes the following changes:

* Delete "Suggested process" section (moved to the Wiki)
* Change the order of "Bug reports" and "Pull requests" so that the latter appears on the first screen
* Add "License" section with a ready to copy template