Share ownership of contribution-related docs between Games and Learning

In the future these will likely move to a separate shared documentation
repository; for now, tag both teams for review.

Author: wjt

.github/CODEOWNERS

@@ -26,6 +26,12 @@ docs/pull_request_template.md    @endlessm/learning
 .github/pull_request_template.md @endlessm/learning
 .github/PULL_REQUEST_TEMPLATE/   @endlessm/learning
 
+# Games and Learning teams share ownership of what's expected of contributions
+# to the game.
+docs/CONTENT_GUIDELINES.md       @endlessm/games @endlessm/learning
+docs/CONTRIBUTING.md             @endlessm/games @endlessm/learning
+docs/REVIEWING.md                @endlessm/games @endlessm/learning
+
 # Community stuff; directly affects the community contributor experience and
 # related documentation
 README.md                 @endlessm/community