Adde README files for each chapter folder

chp3/README.md

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+# Chapter 3 – Buffered I/O
+
+## Difference Between Byte-Level and Buffered I/O
+- **System calls** like `read()` and `write()` operate at a low level: each call may result in direct disk access.
+- **C libraries** use buffered I/O (`fread()`, `fwrite()`, `fprintf()`), accumulating data in memory to reduce system calls.
+
+## Buffered I/O API
+- Uses `FILE *` type from `<stdio.h>`.
+- Functions:
+  - `fopen()`, `fclose()`: open/close file.
+  - `fread()`, `fwrite()`: binary I/O.
+  - `fprintf()`, `fscanf()`, `fgets()`, `fputs()`: formatted or text I/O.
+
+## Types of Buffering
+- **Fully buffered**: flush when buffer is full.
+- **Line buffered**: flush at end of line (e.g., terminals).
+- **Unbuffered**: no accumulation (e.g., stderr).
+
+## Buffer Management
+- `setvbuf()` or `setbuf()` allows custom buffering.
+- Useful for optimizing performance or interactive environments.
+
+## When to Use Buffered I/O
+- When performance matters and immediate file system synchronization isn't required.
+- For formatted I/O, the C library offers more simplicity and readability.
+