Added exercise for signal handler

exercises/signal_handler/README.md

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+# Signal Handler
+
+## Overview
+
+This project demonstrates how to handle UNIX signals in C using `sigaction`.  
+It specifically intercepts **SIGINT** and **SIGUSR1** and prints messages when these signals are received.
+
+---
+
+## Features
+
+- Intercepts **SIGINT** (usually generated with `Ctrl + C`) and **SIGUSR1**`.
+- Prints a custom message for each signal.
+- Tracks the number of `SIGINT` signals received and terminates after a defined count.
+- Demonstrates proper use of:
+  - `sigaction()` instead of `signal()` for consistent behavior.
+  - `volatile sig_atomic_t` for safe signal state handling.
+
+---
+
+## Build
+
+```bash
+make
+```
+
+---
+
+## Run
+
+```bash
+./signal_handler
+```
+
+---
+
+## Test
+
+- Open another terminal and run:
+
+```bash
+kill -USR1 <PID>
+```
+
+where `<PID>` is the process ID of `signal_handler`.
+
+- Press `Ctrl + C` in the terminal running the program to send `SIGINT`.
+
+---
+
+## Clean
+
+```bash
+make clean
+```