message: Add contributing guide.
diff: +# Maintainers
  +This repo is currently maintained by Ken Liao (@yknl).
  +
  +# Pull Request Reviewers
  +Hank Stoever (@hstove)
  +Aaron Blankstein (@kantai)
  +Matthew Little (@zone117x)
  +
  +# Submitting Pull Requests
  +This repository uses the [git flow branching mode](http://nvie.com/posts/a-successful-git-branching-model/).
  +
  +The latest released code as deployed to npm is in `master` and the latest delivered development
  +changes for the next release are in `develop`.
  +
  +We use the [git-flow-avh](https://github.com/petervanderdoes/gitflow-avh) plugin.
  +
  +Pull request requirements:
  +
  +1. Describe exactly what the goal of the PR is (and link to any relevant issues)
  +2. Describe how that goal was achieved through the submitted implementation.
  +3. The code must be lint-free and pass the unit tests by running `$ npm run test`
  +4. The code should contain [flow static type annotations](https://flow.org)
  +5. Contain tests that cover any and all new functionality or code changes.
  +6. Describe how the new functionality can be tested manually.
  +7. Document any new features or endpoints, and describe how developers would be expected to interact with them.
  +8. PR authors should agree to our contributor's agreement.
  +
  +Github issues marked [help-wanted](https://github.com/blockstack/blockstack.js/labels/help-wanted)
  +are great places to start. Please ask in a github issue or slack before embarking
  +on larger issues that aren't labeled as help wanted or adding additional
  +functionality so that we can make sure your contribution can be included!
  +
  +# Reviewing Pull Requests
  +A PR reviewer is responsible for ensuring the following:
  +
  +1. All code changes are covered by automated tests.
  +
  +2. Authentication and Gaia storage continues to work properly when integrated in both the Blockstack Browser and apps.
  +
  +3. Compatibility breaking changes are documented.
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