Update CONTRIBUTING doc.

Author: yoichiro

CONTRIBUTING.md

@@ -3,9 +3,10 @@
 ## Contributing a patch
 
 1. Submit an issue describing your proposed change to the repository in question.
-2. The repository owner will respond to your issue promptly.
-3. If your proposed change is accepted, fork the desired repository, develop and test your code change.
-4. Ensure that your code adheres to the existing style in the code set to which you are contributing.
-5. Ensure that your code has an appropriate set of unit tests which all pass, if possible.
-6. Execute `yarn format` command to format all codes.
-7. Submit a pull request.
+2. This repository owners will respond to your issue promptly.
+3. If your proposed change is accepted, you need to propose a technical design to implement your idea. You can go the next step after the design is accepted by this repository owners.
+4. Fork the desired repository, develop and test your code change.
+5. Ensure that your code adheres to the existing style in the code set to which you are contributing.
+6. Ensure that your code has an appropriate set of unit tests which all pass, if possible.
+7. Execute `yarn format` command to format all codes.
+8. Submit a pull request.