Added contributor guidelines

#155

CONTRIBUTING.md

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+# Contribution guidelines
+
+First off, thank you for contributing to `tlsn`.
+
+If your contribution is not straightforward, please first discuss the change you
+wish to make by creating a new issue.
+
+## Reporting issues
+
+Before reporting an issue on the
+[issue tracker](https://github.com/tlsnotary/tlsn/issues),
+please check that it has not already been reported by searching for some related
+keywords.
+
+## Pull requests
+
+Try to do one pull request per change.
+
+### Updating the changelog
+
+Update the changes you have made in
+[CHANGELOG](CHANGELOG.md)
+file under the **Unreleased** section.
+
+Add the changes of your pull request to one of the following subsections,
+depending on the types of changes defined by
+[Keep a changelog](https://keepachangelog.com/en/1.0.0/):
+
+- `Added` for new features.
+- `Changed` for changes in existing functionality.
+- `Deprecated` for soon-to-be removed features.
+- `Removed` for now removed features.
+- `Fixed` for any bug fixes.
+- `Security` in case of vulnerabilities.
+
+If the required subsection does not exist yet under **Unreleased**, create it!