doc: Contribution guidelines

.github/issue_template.md

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+## Issue summary
+
+[Put a detailed description of the issue here.]
+
+## System information
+
+- [Operating system]
+- [Python version]
+- [Gym Retro version]

CONTRIBUTING.md

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+# Gym Retro Contribution Guidelines
+
+At this time we are currently accepting the current forms of contributions:
+
+- Bug reports in either the core functionality or game integrations
+- Pull requests for core functionality bug fixes
+
+Notably, we are not accepting these forms of contributions:
+
+- New game integrations
+- New features
+
+This may change in the future.
+In the meantime if you wish to integrate new games you are more than welcome to maintain unofficial repositories of additional games.
+
+## Issue reports
+
+Please include the following information in your issue reports:
+
+- Operating system
+- Python version
+- Gym Retro version or git commit
+- A detailed description of the issue
+
+## Code contributions
+
+Please try to adhere to the existing code style. There is a linter script included at `scripts/lint.sh`.
+
+### Python
+
+Gym Retro is written in a [PEP 8-compliant code style](https://www.python.org/dev/peps/pep-0008/) (minus the line length restriction). Please make sure to maintain this style in any contributions.
+
+### C++
+
+There is a `.clang-format` file that documents as best as possible the code style for Gym Retro. Please make sure to follow it.