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]
diff --git a/.gitignore b/.gitignore
index a7b557a..940c2df 100644
--- a/.gitignore
+++ b/.gitignore
@@ -54,6 +54,7 @@
!/LICENSE
!/Makefile
!/README
+!/CONTRIBUTING
!/ROADMAP
!/SUBVERS
!/VERDATE