CODE_OF_CONDUCT: Rename CONTRIBUTING.md to CODE_OF_CONDUCT.md

The current CONTRIBUTING.md file is actually a code of conduct. This is
a valuable document, but is misnamed. A following patch will replace
CONTRIBUTING.md with a guide to contributing to Git using a Windows machine.

Signed-off-by: Derrick Stolee <[email protected]>

Author: derrickstolee