Update readme to add first page indicationg being 1 and not 0

When you request the donations from page 0 or 1, it gives the same items.

Shouldn't zero return a 4xx HTTP code? 
Or at least, please update the documentation to better represent this.


`https://donorbox.org/api/v1/donations?page=0&per_page=100&date_from=2025-03-28&date_to=2025-07-05` === `https://donorbox.org/api/v1/donations?page=1&per_page=100&date_from=2025-03-28&date_to=2025-07-05`