Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing Guidelines
+
+Contributions welcome!
+
+**Before spending lots of time on something, ask for feedback on your idea first!**
+
+Please search issues and pull requests before adding something new to avoid duplicating efforts and conversations.
+
+This project welcomes following types of contributions:
+
+- **Ideas**: participate in an issue thread or start your own to have your voice heard.
+- **Writing**: contribute your expertise in an area by helping expand the included content.
+- **Copy editing**: fix typos, clarify language, and generally improve the quality of the content.
+- **Formatting**: help keep content easy to read with consistent formatting.