Add contributing.md (#326)

Author: s0ph1e

CONTRIBUTING.md

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+## Contributing to Website Scraper
+
+Thank you for considering contributing to Website Scraper! 🎉👍
+
+We love contributions from everyone. :heart:
+
+### Get in touch
+* Report bugs, suggest features, ask usage questions or view the source code on [Github](https://github.com/website-scraper/node-website-scraper)
+* Discuss topics on [Gitter](https://gitter.im/website-scraper/node-website-scraper)
+
+### Issues
+If you've found a bug or have a new feature to suggest - let us know.
+
+#### Reporting Bugs
+When you are creating a bug report, please include as many details as possible. Fill out the [required template](https://github.com/website-scraper/node-website-scraper/blob/master/.github/ISSUE_TEMPLATE.md), the information it asks for helps us resolve issues faster.
+
+#### Suggesting Enhancements
+If you have an idea that would be nice to have, create an issue. Provide a clear and detailed explanation of the feature you want and why it's important to add.
+
+### Pull requests
+
+0. If you are going to work on something large or complex - please create issue for it first and wait until maintainers accept it to ensure your idea is aligned with product vision.
+1. In almost all of the cases your PR needs tests. Make sure you have any.
+2. Run `npm test` locally. Fix any errors before pushing to GitHub.
+3. After submitting the PR a build will be triggered on TravisCI. Wait for it to ends and make sure all jobs are passing.
+4. Don't forget to update documentation (README.md) if you add new option.
+
+### Code of Conduct
+
+All members of our community are expected to follow our [Code of Conduct](https://github.com/website-scraper/node-website-scraper/blob/master/CODE_OF_CONDUCT.md). Please make sure you are welcoming and friendly in all of our spaces.