Update README.md for Project Euler (#3256)

* Update README.md for Project Euler

* Add link to solution template

* Add newlines for better separation

* Add __name__ == __main__ block in template

* Apply suggestions from code review

Co-authored-by: John Law <johnlaw.po@gmail.com>

* Improve introduction part

Co-authored-by: John Law <johnlaw.po@gmail.com>