Provide an explanation about relations with Statistics.jl

[aborzunov]

For first-time visitors, it's easy to get confused between the two packages. Judging by the names, it might seem that statsBase should be the basis for statistics, although the latter package obviously has more extensive functionality.

It is really easy to confuse, and when you start to search you may fall through discussion that leads to PR, that later leads to discussion to to revert those changes. Two line expiation in readme and documentation would make package more friendly for beginners.