docs: Reference the code of conduct doc in the contributing doc

Potential contributors need to be aware of and agree to the projects
code of conduct before making contributions.

Fixes #535.

Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>

CONTRIBUTING.md

@@ -3,6 +3,10 @@
 The Clear Containers runtime is an open source project licensed under the
 [Apache License, Version 2.0](https://www.apache.org/licenses/LICENSE-2.0).
 
+## Code of Conduct
+
+All contributors must agree to the project [code of conduct](CODE_OF_CONDUCT.md).
+
 ## Coding Style (Go)
 
 The usual Go style, enforced by `gofmt`, should be used. Additionally, the [Go