Update README with LeetCode Daily Challenge

Author: jeremymanning

README.md

@@ -427,6 +427,7 @@ _**Note:** Our group is currently taking a break from grinding leetcode. The tab
 | August 18, 2025 | [679](https://leetcode.com/problems/24-game/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/679) | 🔴 Hard |
 | August 19, 2025 | [2348](https://leetcode.com/problems/number-of-zero-filled-subarrays/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/2348) | 🟡 Medium |
 | August 20, 2025 | [1277](https://leetcode.com/problems/count-square-submatrices-with-all-ones/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/1277) | 🟡 Medium |
+| August 21, 2025 | [1504](https://leetcode.com/problems/count-submatrices-with-all-ones/description/?envType=daily-question) | [Click here](https://github.com/ContextLab/leetcode-solutions/tree/main/problems/1504) | 🟡 Medium |
 
 # Join our discussion!