Add CONTRIBUTING.md file (#890)

CONTRIBUTING.md

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+# Contributing
+
+There are many ways that you can contribute to the Azure OpenAPI Validation Tools project:
+
+- Submit a bug
+- Submit a code fix for a bug
+- Submit additions or modifications to the documentation
+- Submit a feature request
+
+All code submissions will be reviewed and tested by the team, and those that meet a high bar for both quality and design/roadmap appropriateness will be merged into the project.
+
+## Getting the Code
+
+Make sure you're running Node.js 14+ and NPM 8+, to verify and upgrade NPM do:
+
+```bash
+node --version
+npm --version
+npm i -g npm@latest
+```
+
+## Clone this repository
+
+```bash
+git clone https://github.com/azure/oav
+cd oav
+```
+
+## Install dependencies
+
+```bash
+npm ci
+```
+
+## Build the project
+
+```bash
+npm run build
+```
+
+## Run the tests
+
+To run all tests in the project
+
+```bash
+npm test
+```
+
+To run all the tests in a particular test file:
+
+```bash
+npm run jest <test-file-name>
+```
+
+## Creating a pull request
+
+Always open one pull request per issue and link the issue in the pull request description.
+If there is no preexisting issue for your pull request, please create one.
+
+To create a pull request, please:
+
+- Create a branch and add your changes. Make sure you include appropriate tests and documentation.
+- Ensure all the tests pass: `npm test`
+- Check for lint errors and fix any that are reported: `npm run lint` and `npm run lint-fix`
+- Commit your changes using a minimum of individual commits. Squash/fixup commits as needed.
+- Push to your fork and submit a pull request to the develop branch. Reference the issue that describes
+the problem or feature and include a summary of your changes, noting any significant design decisions.
+- Reviewers will be added to your PR automatically and if the PR is approved will merge it.
+
+## Code of Conduct
+
+This project has adopted the [Microsoft Open Source Code of Conduct](https://opensource.microsoft.com/codeofconduct/).
+For more information see the [Code of Conduct FAQ](https://opensource.microsoft.com/codeofconduct/faq/) or contact
+[opencode@microsoft.com](mailto:opencode@microsoft.com) with any additional questions or comments.