Add image in `README.md` to show an example of invoice generated

[ali-sajjad-rizavi]

An image will be helpful for people to have an idea what kind of invoice can be generated.

[Mohit-205]

Please provide a screenshot of the program for reference purposes

[PetrDlouhy]

Hi @ali-sajjad-rizavi, @Mohit-205 I am maintainer of this project, but I don't use it any more. If you provide such image or better make a PR with it, I will add it to README/merge it.