Update CONTRIBUTING.md

.github/CONTRIBUTING.md

@@ -1,7 +1,9 @@
 ## License
 
+All contributions are subject to the [Developer Certificate of Origin(DCO)](https://developercertificate.org/). Text of DCO is also included in [DCO.txt](https://github.com/redisson/redisson/blob/master/DCO.txt) file.
+
 All contributions to Redisson are licensed under the
-[Apache License 2.0](https://www.apache.org/licenses/LICENSE-2.0) and subject to the [Developer Certificate of Origin (DCO)](https://developercertificate.org/).
+[Apache License 2.0](https://www.apache.org/licenses/LICENSE-2.0) and 
 
 ## Build Prerequisites
 Have at least a local copy of built redis, for more information see [tutorial](https://www.digitalocean.com/community/tutorials/how-to-install-and-use-redis).