DOC: add a CONTRIBUTING file

This file tries to explain in the most detailed way how to contribute
patches. A few parts of it were moved from the README. .gitignore was
updated.
(cherry picked from commit 11e334d972664c6964e3bac23c7da31c367bfbcb)

[It will help to have this one in 1.5 as well to help bug reporters]
3 files changed