Update README with LeetCode Daily Challenge

Author: jeremymanning

README.md

@@ -93,6 +93,7 @@ _**Note:** Our group is currently taking a break from grinding leetcode. The tab
 | September 18, 2024 | [179](https://leetcode.com/problems/largest-number/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/179) | 🟡 Medium |
 | September 19, 2024 | [241](https://leetcode.com/problems/different-ways-to-add-parentheses/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/241) | 🟡 Medium |
 | September 20, 2024 | [214](https://leetcode.com/problems/shortest-palindrome/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/214) | 🔴 Hard |
+| September 21, 2024 | [386](https://leetcode.com/problems/lexicographical-numbers/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/386) | 🟡 Medium |
 
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