Create CONTRIBUTING.md

Author: kseo

CONTRIBUTING.md

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+If you would like to contribute to CodeChain, please fork it, fix bugs or implement features, and [propose a pull request](https://github.com/CodeChain-io/codechain/compare).
+
+# Report bugs
+
+Create a [new issue](https://github.com/CodeChain-io/codechain/issues/new) in our repository and state:
+
+* What's your CodeChain version?
+* What's your operating system and version?
+* How did you install CodeChain?
+* Is your node fully synchronized?
+* Did you try turning it off and on again?
+* Also, try to include steps to reproduce the issue and expand on the **actual** versus **expected** behavior.
+
+# License
+
+By contributing to CodeChain, you agree that your contributions will be licensed under the AGPLv3 License.
+
+Each contributor has to [sign our Contributor License Agreement](https://www.clahub.com/agreements/CodeChain-io/codechain). The purpose of the CLA is to ensure that the guardian of a project's outputs has the necessary ownership or grants of rights over all contributions to allow them to distribute under the chosen license. You can read and sign our full Contributor License Agreement before submitting a pull request.