Update Readme and instructions

[SporadicThought]

$ apt-get install --assume-yes libsodium libsodium-dev
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package libsodium-dev is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
libsodium

E: Package 'libsodium-dev' has no installation candidate