Update required fields for README

[furliq]

https://github.com/adamniedzielski/tiddle/blob/master/README.md#usage

expires_in is now a required field

body, last_used_at, ip_address and user_agent fields are required.

Needs "expires_in" as well

[alejandrodevs]

@Fredism I was checking tiddle code and expires_in field is not required.

[adamniedzielski]

Yes, it's not required 😄