Create CONTRIBUTING.md

Author: tarolling

.github/CONTRIBUTING.md

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+# How to Contribute
+
+If you want to implement new methods, follow this general format.
+
+- Method names must match the LeetCode problem name in camel case, e.g. Two Sum -> twoSum
+- Method definitions must match the LeetCode definitions provided, types and all (excluding externally defined types like ListNode)
+- Test names must be named the same, prefixed with `test_`
+- Test cases must match the test cases given by LeetCode
+
+Below are features/documentation that may be implemented in future release, but are not deemed a priority as of now.
+
+- Method documentation
+- Assertions to validate method inputs
+- Extensive test cases/full code coverage