Add custom instructions in riscv

[Ορφέας Περδικούρης <el15081@xxxxxxxxxxxxxxx>]

Hello
I am looking for a complete guide for modifying the binutils files in 
order to add custom instructions to riscv.I have installed a riscv 
toolchain.After googling i found some pages suggest manualy modifying 
the binutils files.Are there some specific steps to follow ?
Thanks !