[Docs update] Add renderInputText prop to props table

[Dual-Ice]

I am faced with problem when I need to format range on dates with custom way ( date1 ~ date2 (UTC) ).
I try to use different ways and hacks but can't estimate the result.
After this I go to source code and find renderInputText prop which you use in text computed value.
With this prop I can do all what I want.
Could you add it to https://github.com/mengxiong10/vue2-datepicker#props ?
I think it helps everyone who have the same problems.