Document custom representations and endianness

[mkroening]

I see you have already renamed inner to repr and added them to the list of bitfield parameters in 204f655. Thanks!

I am not sure if you are fine with referring to the endian-num crate here or if the examples should be standalone. I can remove any mention of the endian-num crate if you want me to. The alternative to using endian-num would be:

#[bitfield(u16, repr = u16, from = u16::to_be, into = u16::from_be)]
// instead of 
#[bitfield(u16, repr = be16, from = be16::from_ne, into = be16::to_ne)]

and then using to_ne_bytes() instead of to_be_bytes().

[wrenger]

As this is only a dev-dependency and a nice example, I would say it's ok. Thanks 👍🏻

[mkroening]

Could you publish a new release containing the new repr attribute option? :)