added contribution guidelines

Signed-off-by: Adrian Catangiu <acatan@amazon.com>

CONTRIBUTING.md

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+# Contributing to versionize
+
+## Contribution Workflow
+
+The versionize repository uses the “fork-and-pull” development model. Follow
+these steps if you want to merge your changes:
+
+1. Within your fork of
+   [versionize](https://github.com/firecracker-microvm/versionize), create a
+   branch for your contribution. Use a meaningful name.
+1. Create your contribution, meeting all
+   [contribution quality standards](#contribution-quality-standards)
+1. [Create a pull request](https://help.github.com/articles/creating-a-pull-request-from-a-fork/)
+   against the master branch of the versionize repository.
+1. Work with your reviewers to address any comments and obtain a
+   minimum of 2 approvals, at least one of which must be provided by
+   [a maintainer](MAINTAINERS.md).
+   To update your pull request amend existing commits whenever applicable and
+   then push the new changes to your pull request branch.
+1. Once the pull request is approved, one of the maintainers will merge it.
+
+## Request for Comments
+
+If you just want to receive feedback for a contribution proposal, open an “RFC”
+(“Request for Comments”) pull request:
+
+1. On your fork of
+   [versionize](https://github.com/firecracker-microvm/versionize), create a
+   branch for the contribution you want feedback on. Use a meaningful name.
+1. Create your proposal based on the existing codebase.
+1. [Create a draft pull request](https://github.blog/2019-02-14-introducing-draft-pull-requests/)
+   against the master branch of the versionize repository.
+1. Discuss your proposal with the community on the pull request page (or on any
+   other channel). Add the conclusion(s) of this discussion to the pull request
+   page.
+
+## Contribution Quality Standards
+
+Most quality and style standards are enforced automatically during integration
+testing. Your contribution needs to meet the following standards:
+
+- Separate each **logical change** into its own commit.
+- Each commit must pass all unit & code style tests, and the full pull request
+  must pass all integration tests.
+- Unit test coverage must _increase_ the overall project code coverage.
+- Document all your public functions.
+- Add a descriptive message for each commit. Follow
+  [commit message best practices](https://github.com/erlang/otp/wiki/writing-good-commit-messages).
+- Document your pull requests. Include the reasoning behind each change.
+- Acknowledge versionize's [Apache 2.0 license](LICENSE) and certify that no
+  part of your contribution contravenes this license by signing off on all your
+  commits with `git -s`. Ensure that every file in your pull request has a
+  header referring to the repository license file.