Add CONTRIBUTING.md

Author: asmeurer

CONTRIBUTING.md

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+Contributions to array-api-compat are welcome, so long as they are [in
+scope](https://data-apis.org/array-api-compat/index.html#scope).
+
+Contributors are encouraged to read through the [development
+notes](https://data-apis.org/array-api-compat/dev/index.html) for the package
+to get full context on some of the design decisions and implementation
+details used in the codebase.