add CONTRIBUTING.md

Signed-off-by: Toshikuni Fukaya <[email protected]>

CONTRIBUTING.md

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+# Contributing Guide
+
+- [Ways to Contribute](#ways-to-contribute)
+- [Find an Issue](#find-an-issue)
+- [Ask for Help](#ask-for-help)
+- [Pull Request Lifecycle](#pull-request-lifecycle)
+- [Development Environment Setup](#development-environment-setup)
+- [Sign Your Commits](#sign-your-commits)
+  - [DCO](#dco)
+- [Pull Request Checklist](#pull-request-checklist)
+
+Welcome! We are glad that you want to contribute to our project!
+
+As you get started, you are in the best position to give us feedback on areas of
+our project that we need help with including:
+
+* Problems found during setting up a new developer environment
+* Gaps in our Quickstart Guide or documentation
+* Bugs in our automation scripts
+
+If anything doesn't make sense, or doesn't work when you run it, please open a
+bug report and let us know!
+
+## Ways to Contribute
+
+We welcome many different types of contributions including:
+
+* New features
+* Builds, CI/CD
+* Bug fixes
+* Documentation
+* Issue Triage
+* Answering questions on GitHub Discussions
+* Communications / Social Media / Blog Posts
+* Release management
+
+Not everything happens through a GitHub pull request. Please [contact us](https://github.com/topolvm/pvc-autoresizer/discussions)
+and let's discuss how we can work together.
+
+## Find an Issue
+
+We have good first issues for new contributors and help wanted issues suitable
+for any contributor. [good first issue](https://github.com/topolvm/pvc-autoresizer/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22) has extra information to
+help you make your first contribution. [help wanted](https://github.com/topolvm/pvc-autoresizer/issues?q=is%3Aopen+is%3Aissue+label%3A%22help+wanted%22) are issues
+suitable for someone who isn't a core maintainer and is good to move onto after
+your first pull request.
+
+Sometimes there won’t be any issues with these labels. That’s ok! There is
+likely still something for you to work on. If you want to contribute but you
+don’t know where to start or can't find a suitable issue, you can ask for an
+issue to work on the [Discussions](https://github.com/topolvm/pvc-autoresizer/discussions).
+
+Once you see an issue that you'd like to work on, please post a comment saying
+that you want to work on it. Something like "I want to work on this" is fine.
+
+## Ask for Help
+
+The best way to reach us with a question when contributing is to ask on:
+
+* The original github issue
+
+## Pull Request Lifecycle
+
+If you want to make a non-trivial change, such as a new feature or a bug fix to break exiting behavior,
+please discuss it first with maintainers on a issue, discussion or a proposal PR you send.
+
+When your PR becomes ready for review, it isn't a draft and all tests pass, we will start
+a review process. Reviewers are selected automatically, so you don't care about them.
+
+If your change is approved by us, we will merge it immediately and it will be
+shipped on the next monthly release.
+
+When there has been no activity for 30 days, the stale bot will label it stale.
+And if there is no activity for another 7 days, it will be closed.
+
+## Development Environment Setup
+
+Our recommended environment is Ubuntu 20.04. Because following steps modify your system globally,
+we suggest preparing a dedicated physical or virtual machine.
+
+1. Download the repository.
+
+    ```console
+    git clone https://github.com/topolvm/pvc-autoresizer.git
+    ```
+
+2. Install the required tools.
+
+    ```console
+    cd pvc-autoresizer
+    make setup
+    ```
+
+3. Make changes you wish.
+
+4. Test your changes.
+
+    ```console
+    # for unit test and lint
+    make test
+
+    # for end-to-end test
+    cd e2e
+    make setup
+    make init-cluster
+    make test
+    ```
+
+## Sign Your Commits
+
+### DCO
+Licensing is important to open source projects. It provides some assurances that
+the software will continue to be available based under the terms that the
+author(s) desired. We require that contributors sign off on commits submitted to
+our project's repositories. The [Developer Certificate of Origin
+(DCO)](https://probot.github.io/apps/dco/) is a way to certify that you wrote and
+have the right to contribute the code you are submitting to the project.
+
+You sign-off by adding the following to your commit messages. Your sign-off must
+match the git user and email associated with the commit.
+
+    This is my commit message
+
+    Signed-off-by: Your Name <[email protected]>
+
+Git has a `-s` command line option to do this automatically:
+
+    git commit -s -m 'This is my commit message'
+
+If you forgot to do this and have not yet pushed your changes to the remote
+repository, you can amend your commit with the sign-off by running 
+
+    git commit --amend -s
+
+## Pull Request Checklist
+
+When you submit your pull request, or you push new commits to it, our automated
+systems will run some checks on your new code. We require that your pull request
+passes these checks, but we also have more criteria than just that before we can
+accept and merge it. We recommend that you check the following things locally
+before you submit your code:
+
+- If your code has breaking changes, please update related documents.
+- If you add a new feature, please add unit or end-to-end tests for it.