Add CONTRIBUTING.md

[twolfson]

Since we are going to be adding this to a shared organization, we should probably add a CONTRIBUTING.md which would include:

• Preferred workflow
• Release process

[twolfson]

The current release process has been listed in #24 (comment)

• Determine whether this is a major, minor, or patch release
  • Versioning documentation: http://semver.org/
• Run npm version with release type
  • For example: npm version patch for a patch release
  • This will increment the package.json version, create a new git commit, and create a new git tag
• Run npm publish to publish to npm
• Run git push && git push --tags to publish to GitHub

To put it altogether:

npm version {{version_type}}
npm publish
git push
git push --tags