Create CONTRIBUTING.md

Author: mbahng

CONTRIBUTING.md

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+## Contributing 
+
+To implement a new functionality in the `aten` library, you must 
+1. Add the class or function header in `aten/src/Tensor.h` 
+2. Add the implementation in the correct file (or create a new one) in `aten./*Tensor/*.cpp`. Make sure to update `aten/bindings/CMakeLists.txt` if needed.
+3. Add its pybindings (if a public function that will be used in `ember`) in `aten/bindings/*bindings.cpp`. Make sure to update `aten/bindings/CMakeLists.txt` if needed. 
+4. Add relevant C++ tests in `aten/test/`.  
+5. Not necessary, but it's good to test it out on a personal script for a sanity check.  
+6. Add to the stub files in `ember/aten/*.pyi`. 
+7. Add Python tests in `test/`. 
+8. If everything passes, you can submit a pull request.