Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -46,10 +46,11 @@ When reporting issues, refer to this format:
 - What you expected to happen?
 - How to reproduce it?(as minimally and precisely as possible)
 
-### Propose blueprints
+### Propose PRs
 
-- Raise your idea as an [issues](https://github.com/opensds/opensds/issues)
-- After reaching consensus in the issue discussion, complete the development on the forked repo and submit a PR. 
+- Raise your idea as an [issue](https://github.com/opensds/opensds/issues)
+- If it is a new feature that needs lots of design details, a design proposal should also be submitted [here](https://github.com/opensds/design-specs/pulls).
+- After reaching consensus in the issue discussions and design proposal reviews, complete the development on the forked repo and submit a PR. 
   Here are the [PRs](https://github.com/opensds/opensds/pulls?q=is%3Apr+is%3Aclosed) that are already closed.
 - If a PR is submitted by one of the core members, it has to be merged by a different core member.
 - After PR is sufficiently discussed, it will get merged, abondoned or rejected depending on the outcome of the discussion.