Document $idx loop index variable in README

[davidjb]

This adds info about the implicit $idx loop index variable into the README.

[jjhbw]

Wow great find! This is AFAIK not intentional behaviour, but we may as well formalise it. The implementation is stable. I'll add some test coverage for it, too.

[jjhbw]

@davidjb Added a test in a3c8c6d and extended the docs a bit with an example. Thanks for noticing this!

[jjhbw]

@davidjb would be great if you could take a last look at a3c8c6d by the way to check if you see anything weird.

[davidjb]

Lgtm @jjhbw - nice work on the tests and extra documentation 👍