Merge pull request #465 from Unity-Technologies/docs/contribution-guidelines

Added Contribution Guidelines

docs/Contribution-Guidelines.md

 # Contribution Guidelines
 
-Reference code of conduct.
+Thank you for your interest in contributing to ML-Agents! We are incredibly
+excited to see how members of our community will use and extend ML-Agents.
+To facilitate your contributions, we've outlined a brief set of guidelines
+to ensure that your extensions can be easily integrated.
-## GitHub Workflow
+### Communication
-## Environments
+First, please read through our [code of conduct](../CODE_OF_CONDUCT.md), 
+as we expect all our contributors to follow it.
+
+Second, before starting on a project that you intend to contribute
+to ML-Agents (whether environments or modifications to the codebase), 
+we **strongly** recommend posting on our 
+[Issues page](https://github.com/Unity-Technologies/ml-agents/issues) and
+briefly outlining the changes you plan to make. This will enable us to provide
+some context that may be helpful for you. This could range from advice and 
+feedback on how to optimally perform your changes or reasons for not doing it.
+
+Lastly, if you're looking for input on what to contribute, feel free to
+reach out to us directly at ml-agents@unity3d.com and/or browse the GitHub
+issues with the `contributions welcome` label.
+
+### Git Branches
+
+Starting with v0.3, we adopted the 
+[Gitflow Workflow](http://nvie.com/posts/a-successful-git-branching-model/).
+Consequently, the `master` branch corresponds to the latest release of 
+the project, while the `develop` branch corresponds to the most recent, stable,
+version of the project.
+
+Thus, when adding to the project, **please branch off `develop`**
+and make sure that your Pull Request (PR) contains the following:
+* Detailed description of the changes performed
+* Corresponding changes to documentation, unit tests and sample environments 
+(if applicable)
+* Summary of the tests performed to validate your changes
+* Issue numbers that the PR resolves (if any)
+
+### Environments
-Pull-Request explaining the nature of the environment and task. 
+PR explaining the nature of the environment and task. 
-TODO: above paragraph needs expansion.
+### Style Guide
-## Algorithms
-
-## Style Guide
-
+When performing changes to the codebase, ensure that you follow the style
+guide of the file you're modifying. For Python, we follow 
+[PEP 8](https://www.python.org/dev/peps/pep-0008/). For C#, we will soon be
+adding a formal style guide for our repository.