Doc update

Updating the documentation for navitia.io release
Note 1: if several kind of journeys are expected, you can request many "first_section_mode[]". For example, "&first_section_mode[]= walking&first_section_mode[]=bss" could provided very different trips in on request.
Note 2: choosing bss implicitly allows the walking mode as a section of the trip since you might have to walk to the bss station