docs: Add CONTRIBUTING

Author: Xuanwo

CONTRIBUTING.md

@@ -0,0 +1,31 @@
+# Contributing to Storage
+
++1tada First off, thanks for taking the time to contribute! tada+1
+
+## Did you find a bug?
+
+- Ensure the bug was not already reported by searching on GitHub under [Issues](https://github.com/Xuanwo/storage/issues).
+- Open a new issue with following things: 
+  - bug description
+  - lib commit id
+  - minimal reproduction code
+
+## Did you write a patch that fixes a bug?
+
+- Open a new GitHub pull request with the patch.
+- Ensure the PR description clearly describes the problem and solution. Include the relevant issue number if applicable.
+- Add unittest for this bug.
+
+## Do you intend to implement a new service?
+
+- `Storager` must be implemented, others can be optional.
+- Add support in `coreutils.Open`.
+- Add unittests as best effort.
+
+## Do you intend to change public API?
+
+- Open a new Github Issue for discuss.
+- After achieve consensus, add a proposal in `docs/design` and submit a PR.
+- Implement a proposal and change status to `candidate`
+
+> In next release, relevant proposal statue will be updated to `finished`