Create contributing.md

Adding contribution instructions, mirroring https://iabtechlab.com/opensource/

contributing.md

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+A **Contributor License Agreement (CLA)** defines the terms under which intellectual property has been contributed to a company/project. The purpose of a CLA is to ensure that IAB Tech Lab has the necessary grants of rights over all contributions to allow them to be distributed under the chosen license.
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+Contributors to the projects will be made under one or more roles as defined below:
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+- **Project Release Manager** is overall responsible for the development of the project and has write access to the repository
+- **Committer** is a developer with write access to the repository and responsible for making final changes.
+- **Developer** is a contributor to the project in form of code or documentation and may take on other responsibilities for managing the project
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+All contributors who wish to or are required to have write access **(Committers)** need to be/do one of the following:
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+Be a member of a Tech Lab member company (no additional paperwork required)
+OR execute the [Contributor License Agreement](https://iabtechlab.com/wp-content/uploads/2021/07/IAB-TL-Contributor-License-Agreement.pdf) 
+To become a **Committer**, please email [[email protected]](mailto:[email protected]) with the link to the project repository to which you’d like to commit as well as the signed copy of the Contributor License Agreement if required.