Added README files for exercises

exercises/tree/README.md

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+# 🗂️ tree – Directory Tree Viewer in C
+
+A simplified implementation of the Unix `tree` command in C.  
+This utility recursively traverses a directory and prints its structure in a tree-like format.
+
+---
+
+## 📦 Project Structure
+
+```
+tree/
+├── main.c           # Entry point
+├── tree.c           # Directory traversal logic
+├── tree.h           # Function declarations
+├── Makefile         # Build & test automation
+└── test/
+    └── run_tests.sh # Automated tests + Valgrind
+```
+
+---
+
+## ⚙️ Build Instructions
+
+To compile the program:
+
+```bash
+make
+```
+
+To clean up build files:
+
+```bash
+make clean
+```
+
+To remove test artifacts as well:
+
+```bash
+make distclean
+```
+
+---
+
+## 🧪 Running Tests
+
+The test script:
+
+- Creates a temporary directory structure
+- Runs the program and compares its output to the expected result
+- Performs memory leak analysis with **Valgrind**
+
+To run all tests:
+
+```bash
+make test
+```
+
+---
+
+## 🚀 Usage
+
+```bash
+./tree [DIRECTORY]
+```
+
+If no directory is specified, the current directory (`.`) is used.
+
+Example:
+
+```bash
+./tree testdir
+```
+
+Sample output:
+
+```
+|-- file1.txt
+|-- sub1
+  |-- file2.txt
+|-- sub2
+  |-- file3.txt
+```
+
+---
+
+## 🧠 Learning Objectives
+
+- Use of `opendir`, `readdir`, and `closedir` to iterate directories
+- Handling file paths and recursion
+- Working with `stat` to detect subdirectories
+- Implementing simple CLI programs in C
+- Using `Makefile` and Valgrind for tooling and quality
+
+---
+
+## ✅ Requirements
+
+- GCC (C11)
+- POSIX-compliant system (Linux, macOS)
+- [Valgrind](https://valgrind.org/) for memory checking (optional, used in tests)
+
+---
+
+## 📄 License
+
+Educational use only. No license required.
+