Problem: C4 process is not easily discoverable.

Solution: Add 'Contributing' section to README.md

README.md

@@ -100,3 +100,7 @@ libzmq is distributed in the hope that it will be useful, but WITHOUT
 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
 License for more details.
+
+## Contributing
+
+This project uses [C4(Collective Code Construction Contract)](https://rfc.zeromq.org/spec:42/C4/) process for contributions.