Add Contributing guidelines to repo.

CONTRIBUTING.md

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+# How to contribute
+
+Your contributions to the Stellar network will help improve the world’s financial
+infrastructure, faster.
+
+We want to make it as easy as possible to contribute changes that
+help the Stellar network grow and thrive. There are a few guidelines that we
+ask contributors to follow so that we can merge your changes quickly.
+
+## Getting Started
+
+* Create a GitHub issue for your contribution, assuming one does not already exist.
+  * Clearly describe the issue including steps to reproduce if it is a bug.
+* Fork the repository on GitHub
+
+## Making Changes
+
+* Create a topic branch from where you want to base your work.
+  * This is usually the master branch.
+  * Please avoid working directly on the `master` branch.
+* Make sure you have added the necessary tests for your changes, and make sure all tests pass.
+
+## Submitting Changes
+
+* <a href="https://docs.google.com/forms/d/1g7EF6PERciwn7zfmfke5Sir2n10yddGGSXyZsq98tVY/viewform?usp=send_form">Sign the Contributor License Agreement</a>.
+* All content, comments, and pull requests must follow the [Stellar Community Guidelines](https://www.stellar.org/community-guidelines/). 
+* Push your changes to a topic branch in your fork of the repository.
+* Submit a pull request to the [stellar_core_commander repository](https://github.com/stellar/stellar-tutorials) in the Stellar organization.
+ * Include a descriptive [commit message](https://github.com/erlang/otp/wiki/Writing-good-commit-messages).
+ * Changes contributed via pull request should focus on a single issue at a time.
+ * Rebase your local changes against the master branch. Resolve any conflicts that arise.
+
+At this point you're waiting on us. We like to at least comment on pull requests within three 
+business days (and, typically, one business day). We may suggest some changes or improvements or alternatives.
+
+## Making Trivial Changes
+
+### Documentation
+For changes of a trivial nature to comments and documentation, it is not
+always necessary to create a new GitHub issue. In this case, it is
+appropriate to start the first line of a commit with 'doc' instead of
+an issue number. 
+
+# Additional Resources
+
+* [Bug tracker (Github)](https://github.com/stellar/stellar_core_commander/issues)
+* <a href="https://docs.google.com/forms/d/1g7EF6PERciwn7zfmfke5Sir2n10yddGGSXyZsq98tVY/viewform?usp=send_form">Contributor License Agreement</a>
+* [Explore the API](http://docs.stellarhorizon.apiary.io/)
+* [Readme for stellar_core_commander](https://github.com/stellar/stellar_core_commander/blob/master/README.md)
+* #stellar-dev IRC channel on freenode.org
+* #dev channel on [Slack](http://slack.stellar.org)
+
+
+This document is inspired by:
+
+https://github.com/puppetlabs/puppet/blob/master/CONTRIBUTING.md 
+
+https://github.com/thoughtbot/factory_girl_rails/blob/master/CONTRIBUTING.md 
+
+https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md