Add missing CODEOWNERS and harden .github path

The whole idea of "hardening" the .github path is to prevent
large teams to have access to that path.

Instead, only a subset of that team must be allowed that.

In our case, we're a team of 3 at Neo4j, so listing us 3 should
be fine, even if the team grows beyond that.

.github/CODEOWNERS

@@ -0,0 +1,4 @@
+*       @neo4j-contrib/team-connectors
+*       @conker84
+
+/.github/   @ali-ince @fbiville @venikkin