Update README.md

Author: Jenni-Foued

0x0D-preprocessor/README.md

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-# Preprocessor
+# 0x0D. C - Preprocessor
+
+## About
+
+   - What are macros and how to use them
+   - What are the most common predefined macros
+   - How to include guard your header files
+
+## Files & Descriptions
+
+[0-object_like_macro.h](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x0D-preprocessor/0-object_like_macro.h)  a header file that defines a macro named SIZE as an abbreviation for the token 1024
+
+[1-pi.h](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x0D-preprocessor/1-pi.h)  a header file that defines a macro named PI as an abbreviation for the token 3.14159265359
+
+[2-main.c](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x0D-preprocessor/2-main.c)   a program that prints the name of the file it was compiled from, followed by a new line
+
+[3-function_like_macro.h](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x0D-preprocessor/3-function_like_macro.h)  a function-like macro ABS(x) that computes the absolute value of a number x
+
+[4-sum.h](https://github.com/Jenni-Foued/holbertonschool-low_level_programming/tree/master/0x0D-preprocessor/4-sum.h)  a function-like macro SUM(x, y) that computes the sum of the numbers x and y
+
+[101-preprocessor_abuse.c] a program that prints a string `Hello, Holberton\n`