Add CONTRIBUTING guidelines

CONTRIBUTING.md

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+# Contribution Guidelines
+
+When making a pull request, please follow these guidelines:
+
+- One commit per suggestion
+- Commit message should follow this format: `Add some tool name`
+- Multiple commits per pull request is OK
+- Lists within each section are alphabetized
+- Add sections if necessary
+- Clear, concise descriptions for each link, followed by a period
+
+Thanks!