This PR updates GitHub template for `CODE-OF-CONDUCT.md => `.github/CODE_OF_CONDUCT.md`,
to take advantage of GitHub feature in:

https://help.github.com/en/github/building-a-strong-community/adding-a-code-of-conduct-to-your-project

This PR also moves `SECURITY.md` to `.github/SECURITY.md` and alias it for consistency.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>