Add a reference CODE_OF_CONDUCT.md file

it is of my opinion that this repository should include a CODE_OF_CONDUCT.md file:
* the CODE_OF_CONDUCT.md file that is posted here can serve as a reference for other EPA repositories to adopt into their own repository
* In addition to a LICENSE.md file, any repository that includes a contributing.md, in my opinion, should also include a CODE_OF_CONDUCT.md file