Documentation issue: add libsodium-dev to dependencies

[ttimmy]

Hi,
On clean Ubuntu 18.04, following the instructions and running "npm i" results in compilation error of missing sodium.h file.
Only by manually sudo apt-get install libsodium-dev, does the issue resolve. It might even require libboost-all-dev, since I followed this issue here: aionnetwork/aion_miner#6

Thx, have a good one

[neofutur]

I also needed to add libsodium, libboost-dev and libboost-system-dev.