Move CODE_OF_CONDUCT.md to the main directory (#2973)

* Move CODE_OF_CONDUCT.md to the main directory

* Add CODE_OF_CONDUCT.md to .Rbuildignore

Author: yutannihilation

.Rbuildignore

@@ -11,6 +11,7 @@ visual_test
 ^CONTRIBUTING\.md$
 ^GOVERNANCE\.md$
 ^ISSUE_TEMPLATE\.md$
+^CODE_OF_CONDUCT\.md$
 ^NEWS$
 ^revdep$
 ^\.httr-oauth$