added README

Author: Mannuel25

Countdown Timer/README.md

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+# Countdown Timer
+
+This program takes in the number of seconds to countdown and displays a message when the time elapses.
+
+# Prerequisites
+
+It requires no prerequisites, you only need to run the script. If you don't have Python installed, you can visit [here](https://www.python.org/downloads/) to download Python.
+
+# How to run the script
+
+Running the script is pretty easy, open a terminal in the folder where your script is located and run the following command :
+
+`python timer.py`
+
+# Sample use of the script
+
+![alt text](https://github.com/Mannuel25/py-projects/blob/master/all-python-codes/countdown-timer/screenshot_1.png)
+
+![alt text](https://github.com/Mannuel25/py-projects/blob/master/all-python-codes/countdown-timer/screenshot_2.png)