Add 'contributing.md'

contributing.md

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+# Contribution Guidelines
+- **To add, remove, or change things on the list:** Submit a pull request
+- List items should be sorted *alphabetically*.
+- Each item should be limited to one link and a description.
+- The link should be the name of the project.
+- Descriptions should be clear, concise, and non-promotional.
+- Descriptions should follow the link, on the same line and end with a punctuation mark.