Update README.md

Author: neetcode-gh

Diff for: articles/README.md

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+# Article Guide
 
+1. Each article should be written in markdown. Exanple: [duplicate-integer.md](https://github.com/neetcode-gh/leetcode/blob/main/articles/duplicate-integer.md)
+2. At least one solution should be similar to a solution present in the NeetCode video. Ideally all solutions from the video are present.
+3. Add time & space complexity
+4. Cover all relevant solutions if possible.