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.
diff --git a/.gitignore b/.gitignore
index 5dde3bf..acdbcc9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -55,6 +55,7 @@
 !/LICENSE
 !/Makefile
 !/README
+!/CONTRIBUTING
 !/ROADMAP
 !/SUBVERS
 !/VERDATE