Add Contribution Guidelines

CONTRIBUTING.md

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+# Contributing to open62541
+
+Contributions to open62541 include code, documentation, answering user
+questions, running the project's infrastructure, and advocating for all types of
+open62541 users.
+
+The open62541 project welcomes all contributions from anyone willing to work in
+good faith with other contributors and the community. No contribution is too
+small and all contributions are valued.
+
+This guide explains the process for contributing to the open62541 project's core
+repository and describes what to expect at each step. Thank you for considering
+these point.
+
+Your friendly open62541 community!
+
+## Code of Conduct
+
+The open62541 project has a [Code of Conduct](./CODE_OF_CONDUCT.md) that *all*
+contributors are expected to follow. This code describes the *minimum* behavior
+expectations for all contributors.
+
+## Issues
+
+- Help us help you
+- Use the [Issue Template](./.github/ISSUE_TEMPLATE)
+
+## Pull Requests
+
+Everybody can propose a pull request (PR). But only the core-maintainers of the
+project can merge PR.
+
+### Minimal requirements for a PR
+
+The following are the minimal requirements that every PR needs to meet.
+
+- **Pass Continuous Integration (CI)**: Every PR has to pass our CI. This
+  includes compilation with a range of compilers and for a range of target
+  architectures, passing the unit tests and no detected issues with static code
+  analysis tools.
+
+- **Code-Style**: Please consider the
+  [Code-Style](https://github.com/open62541/open62541/wiki/Code-Style)
+  recommendations when formatting your code.
+
+- **Signed CLA**: Every contributor must sign the Contributor License Agreement
+  (CLA) before we can merge his first PR. The signing can be done online. The
+  link automatically appears on the page of the first PR. In addition, the CLA
+  text can be accessed [here](https://cla-assistant.io/open62541/open62541).
+
+### Commit and PR Hygiene
+
+- **Separation of Concerns**: Small changes are much easier to review.
+  Typically, small PR are merged much faster. For larger contributions, it might
+  make sense to break them up into a series of PR. For example, a PR with a new
+  feature should not contain other commits with only stylistic improvements to
+  another portion of the code.
+
+- **Feature Commits**: The same holds true for the individual PR as well. Every
+  commit inside the PR should do one thing only. If many changes have been
+  applied at the same time, `git add --patch` can be used to partially stage and
+  commit changes that belong together.
+
+- **Commit Messages**: Good commit messages help in understanding changes.
+  Consider the following article with best practices for commit messages:
+  https://chris.beams.io/posts/git-commit
+
+- **Linear Commit History**: Our goal is to maintain a linear commit history
+  where possible. Use the `git rebase` functionality before pushing a PR. Use
+  `git rebase --interactive` to squash bugfix commits.
+
+### Review Process
+
+The following labels can be used for the PR title to indicate its status.
+
+- [WIP]: The PR is work in progress and at this point simply informative.
+- [Review]: The PR is ready from the developers perspective. He requests a review from a core-maintainer.
+
+The core-maintainers are busy people. If they take especially long to react,
+feel free to trigger them by additional comments in the PR thread. Again, small
+PR are much faster to review.
+
+It is the job of the developer that posts the PR to rebase the PR on the target
+branch when the two diverge.
+
+### Changes to the public API
+
+The *public* API is the collection of header files in the /include folder.
+
+Changes to the public API are under especially high scrutiny. Public API changes
+are best discussed with the core-maintainers early on. Simply to avoid duplicate
+work when changes to the proposed API become necessary.
+
+You can create a special issue or PR just for the sake of discussing a proposed
+API change. The actual implementation can follow later on.