Document [] isAutoincrement syntax for names.

[KES777]

Describe the bug
This is actually not a bug, but a lack of documentation. During debug session I found that [] is the special syntax used by the library.

In my code I had:

%= hidden_field 'Company[Email][][id]' => ''
%= form_field  input => 'Email', 'Company[Email][][email]',

I resolved my problem just by replacing [] to [X]. The debug session below is just for information purposes.

Debug session

During debugging I found that library supports [] isAutoincrement. But I did not find any documentation. Though I can see only one example without explanation here https://brainfoolong.github.io/form-data-json/

But it is for <select> and it is not clear how it works for the array of inputs.