Update README.md

Updates README with a todo list.

Author: self-init

README.md

@@ -1,2 +1,9 @@
 # Python GUI Calculator with CI/CD
 A simple gui calculator app written in Python and Tkinter that integrates a CI/CD pipeline.
+
+TODO
+ - [x] Basic calculator functionality
+ - [ ] Implement % button
+ - [ ] Multiple layouts & dynamic scaling
+ - [ ] Tests and documentation
+ - [ ] CI/CD workflow using actions