Added developer setup instructions.

Author: j-rewerts

README.md

@@ -94,3 +94,33 @@ npm install github-api
 [npm-package]: https://www.npmjs.com/package/github-api/
 [unpkg]: https://unpkg.com/github-api/
 [travis-ci]: https://travis-ci.org/github-tools/github
+
+## Contributing
+
+We welcome contributions of all types! This section will guide you through setting up your development environment.
+
+### Setup
+
+1. [Install Node](https://nodejs.org/en/) version 8,10 or 11. It can often help to use a Node version switcher such as [NVM](https://github.com/nvm-sh/nvm).
+2. Fork this repo to your GitHub account.
+3. Clone the fork to your development machine (`git clone https://github.com/{YOUR_USERNAME}/github`).
+4. From the root of the cloned repo, run `npm install`.
+5. Email jaredrewerts@gmail.com with the subject **GitHub API - Personal Access Token Request**
+
+A personal access token for our test user, @github-tools-tests, will be generated for you.
+
+6. Set the environment variable `GHTOOLS_USER` to `github-tools-tests`.
+
+`export GHTOOLS_USER=github-tools-tests`
+
+7. Set the environment variable `GHTOOLS_PASSWORD` to the personal access token that was generated for you.
+
+`export GHTOOLS_PASSWORD={YOUR_PAT}`
+
+**NOTE** Windows users can use [this guide](http://www.dowdandassociates.com/blog/content/howto-set-an-environment-variable-in-windows-command-line-and-registry/) to learn about setting environment variables on Windows.
+
+### Tests
+
+The main way we write code for `github-api` is using test-driven development. We use Mocha to run our tests. Given that the bulk of this library is just interacting with GitHub's API, nearly all of our tests are integration tests. 
+
+To run the test suite, run `npm run test`.