More updates to USAGE and README

[freakboy3742]

• Adds details on python-config and testbed
• Adds a bare bones interpreter instantiation example
• Clarifies the difference between instantiating the interpreter, and accessing the Python API with PythonKit

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[frogcjn]

date time should be explicit include as submodule, since the umbrella header did not import it, and most developer may need this datetime module.

If you add datetime in the module map, developers could import datetime as submodule like import Python.datetime to use it.

    explicit module datetime {
      header "datetime.h"
      export *
    }

I also suggest to include these submodules

    explicit module datetime {
      header "datetime.h"
      export *
    }

    explicit module errcode {
      header "errcode.h"
      export *
    }

    explicit module frameobject {
      header "frameobject.h"
      export *
    }

    explicit module marshal {
      header "marshal.h"
      export *
    }

    explicit module opcode {
      header "opcode.h"
      export *
    }

    explicit module osdefs {
      header "osdefs.h"
      export *
    }

    explicit module py_curses {
      header "py_curses.h"
      export *
    }

    explicit module pythread {
      header "pythread.h"
      export *
    }

    explicit module structmember {
      header "structmember.h"
      export *
    }

    explicit module structseq {
      header "structseq.h"
      export *
    }
    ```
    

[freakboy3742]

Once again - commenting on a closed ticket doesn't really help move a discussion forward. If you think there is something additional required, please open a ticket and/or a pull request to add the change you are proposing.

In this specific case - you may also want to add why the change is needed. I understand that import Python.datetime won't be possible... but why should it be? What decision process has driven the 10 specific explicit exports you've added here?