Create CONTRIBUTING.md

Author: ljeng

CONTRIBUTING.md

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+If you're creating a new file, use this template as your header:
+
+````
+## file-name
+````
+
+To contribute a new method, please follow this template:
+
+````
+### [file_name.**method**(*arg*, *kwarg=default*)](/file_name.py)
+
+Description of the method.
+
+[LeetCode Problem Name](https://leetcode.com/problems/leetcode-problem-link)
+```python
+def defaultMethodName(args):
+    # Write your code here
+```
+````
+
+To contribute a new class, please follow this template:
+
+````
+### [class file_name.**ClassName**(*arg*, *kwarg*)](/file_name.py)
+
+Description of the class.
+
+**method_within_class**(*arg*, *kwarg*)
+
+Description of the method.
+
+[LeetCode Problem Name](https://leetcode.com/problems/leetcode-problem-link)
+```python
+def defaultMethodName(args):
+    # Write your code here
+```
+````