README needs some updating

[Ultimater]

The README shows if(empty(session_id();)) session_start(); and contains an extra semicolon surrounded by parenthesis which will cause a syntax error. This can cost people a lot of time trying to find the error, thinking it's the code they added, not realizing the documentation put a stumbling block in front of them.

Probably could update some other things in the documentation as well.
Maybe mention version 1.5 requires PHP 7.3+ and supports PHP 8.
And maybe mention how to use $infusionsoft->refreshAccessToken() once they have storage set up.