tkinter: Update README.md

Simple Calculator Program using PYTHON (tkinter)
1. takes two number from user as input
2. Performs action (+, *, -, /) according to user choice
3. Shows result at the bottom of the window.

python: Update README.md

Simple Calculator Program using PYTHON (tkinter)
1. takes two number from user as input
2. Performs action (+, *, -, /) according to user choice
3. Shows result at the bottom of the window.