docs: add problem description for 76

Author: bujosa

src/hard/readme.md

@@ -31,6 +31,54 @@ Put the code below in main.rs and run `cargo run`
     println!("result: {:?}", result);
 ```
 
+# 76. Minimum Window Substring
+
+## Description
+
+Given two strings s and t of lengths m and n respectively, return the minimum window substring of s such that every character in t (including duplicates) is included in the window. If there is no such substring, return the empty string "".
+
+The testcases will be generated such that the answer is unique.
+
+A substring is a contiguous sequence of characters within the string.
+
+## Examples
+
+Example 1:
+
+```
+Input: s = "ADOBECODEBANC", t = "ABC"
+Output: "BANC"
+Explanation: The minimum window substring "BANC" includes 'A', 'B', and 'C' from string t.
+```
+
+Example 2:
+
+```
+Input: s = "a", t = "a"
+Output: "a"
+Explanation: The entire string s is the minimum window.
+```
+
+Example 3:
+
+```
+Input: s = "a", t = "aa"
+Output: ""
+Explanation: Both 'a's from t must be included in the window.
+Since the largest window of s only has one 'a', return empty string.
+```
+
+## How to Run in main.rs
+
+Put the code below in main.rs and run `cargo run`
+
+```rust
+    let s = String::from("ADOBECODEBANC");
+    let t = String::from("ABC");
+    let result = hard::minimum_window_substring::min_window(s, t);
+    println!("result: {:?}", result);
+```
+
 # 84. Largest Rectangle in Histogram
 
 ## Description