improve user-friendliness of README file

Good suggestion from one of our SoftwareX reviewers:

> Currently, the README has a mix of user-friendly information and technical information, and the technical information may be intimidating to new users. This problem can be easily avoided by having a quick start section in the README and a user guide page that are focused only on the features that most users will need and moving the technical information to a separate page or pages.
>
> For example, currently the "User guide" link in the TOC goes to a section on the smuggle statement, which starts with the grammar definition using BNF notation. Most users will have no idea what those symbols mean, so the BNF won't be very helpful for most users and may be intimidating to people who aren't serious Python developers.