Add CONTRIBUTING.md doc

CONTRIBUTING.md

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+# Contributing
+
+Thanks for being willing to contribute!
+
+**Working on your first Pull Request?** You can learn how from this _free_
+series [How to Contribute to an Open Source Project on GitHub][egghead]
+
+## Project setup
+
+1.  Fork and clone the repo
+2.  Run `npm install` to install dependencies
+3.  Create a branch for your PR with `git checkout -b pr/your-branch-name`
+
+> Tip: Keep your `master` branch pointing at the original repository and make
+> pull requests from branches on your fork. To do this, run:
+>
+> ```
+> git remote add upstream https://github.com/manifoldco/swagger-to-ts.git
+> git fetch upstream
+> git branch --set-upstream-to=upstream/master master
+> ```
+>
+> This will add the original repository as a "remote" called "upstream," Then
+> fetch the git information from that remote, then set your local `master`
+> branch to use the upstream master branch whenever you run `git pull`. Then you
+> can make all of your pull request branches based on this `master` branch.
+> Whenever you want to update your version of `master`, do a regular `git pull`.
+
+## Committing and Pushing changes
+
+Please make sure to run the tests (`npm test`) before you commit your changes.
+
+## Help needed
+
+Please checkout the [the open issues][issues]
+
+Also, please watch the repo and respond to questions/bug reports/feature
+requests! Thanks!
+
+[all-contributors]: https://github.com/all-contributors/all-contributors
+[egghead]: https://egghead.io/series/how-to-contribute-to-an-open-source-project-on-github
+[issues]: https://github.com/manifoldco/swagger-to-ts/issues