Add note to NuGet README about requiring VC2015 redist

[jacobkiers]

During deployment of libsodium.dll (via Nuget, https://www.nuget.org/packages/libsodium/), I found out that the VC2015 redistributable binary is required. This is not listed on the README or in the list of dependencies there. Would it be possible to add this in the NuGet README?

I have added it here, as it is not a bug in functionality. But if documentation issues are considered bugs as well, I am happy to add it to the issue tracker as well.

Thanks in advance!