feat: add docs for 42 problem

Author: bujosa

src/hard/readme.md

@@ -1,3 +1,36 @@
+# 42. Trapping Rain Water
+
+## Description
+
+Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining.
+
+## Examples
+
+Example 1:
+
+```
+Input: height = [0,1,0,2,1,0,1,3,2,1,2,1]
+Output: 6
+Explanation: The above elevation map (black section) is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped.
+```
+
+Example 2:
+
+```
+Input: height = [4,2,0,3,2,5]
+Output: 9
+```
+
+## How to Run in main.rs
+
+Put the code below in main.rs and run `cargo run`
+
+```rust
+    let height = vec![0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1];
+    let result = hard::trapping_rain_water::trap(height);
+    println!("result: {:?}", result);
+```
+
 # 84. Largest Rectangle in Histogram
 
 ## Description