Added libray used in the program.

The Tkinter library which was used to make the game is added in the Requirements section of readme file.

Author: NishaantKrSingh

Flappy Bird - created with tkinter/README.md

@@ -4,5 +4,9 @@ Enjoy and do give feedback <br/>
 Warning: If the game is too slow, lower the game resolution in Data/settings.json
 
 # Requirements:
+Tkinter 
+```bash
+pip install tkinter
+```
 PIL <br />
 Python 3.x