Consider adding a warning about lengthy work

[dlesl]

Rustler is quite mature now and thanks to its great documentation it's easy to get a project working without reading the official erl_nif docs, which unfortunately means that users might not be aware of the big warning box there. While rustler can help eliminate memory errors, the warning about lengthy work is still just as valid, and for many projects, finding a way to do lengthy work correctly will end up being the most challenging part of writing a NIF. I think it would be helpful to:

• Add a warning to the rustler readme with a link to the erl_nif docs
• Link to the ways rustler supports the strategies mentioned in the erl_nif docs
  • Threaded NIFs
  • Dirty NIFs
  • And Yielding NIFs once schedule_nif improvements #232 lands

[dlesl]

enif_consume_timeslice would be good to mention too

[scrogson]

Agreed.