update README for github migration

Let's use git-tag to release the software, then we can get
zip/tar.gz files from https://github.com/crook/yum-axelget/releases

Github issue page is also a good channel for bug report/discussion.
https://github.com/crook/yum-axelget/issues

This patch also added "Contribution" in README.md

Signed-off-by: Amos Kong <kongjianjun@gmail.com>