Added Contribution Guide

Author: Shreyas Padhye

CONTRIBUTING.md

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+# Contributing
+When contributing to this repository, please first discuss the change you wish to make via issue, email, or any other method with the owners of this repository before making a change.
+
+Please note we have a [code of conduct](https://microsoft.github.io/codeofconduct/), please follow it in all your interactions with the project.
+
+## Pull Request Process
+1. Fork repository and create your feature branch out of master.
+2. Open [Codegen project](./package/Codegen/Codegen.csproj) in Visual Studio.
+3. Right click on project in solution explorer -> select "Manage Nuget Packages". This should open the NuGet Manager which will prompt you to restore NuGet packages. Click on "Restore". Once you restore the packages, the project is ready to build / run.
+4. If you make changes in a .tt file, all you need to do is save the file and that will generate corresponding files that reflect your change.
+5. For all other changes: run `yarn build` in src folder so that related files are generated
+6. Run from Visual Studio to see output.
+7. Once changes are ready, run `yarn change` at the root of the repository to create a patch (it will ask for a description of the change). The version scheme we use is [SemVer](https://semver.org/)
+8. Open a pull request and address review feedback.
+
+
+## Technical details:
+You can find more details to setting up in the [Technical Guide](TechnicalGuide.md).