update readme: not 7x faster than Base.log

[floswald]

i think it might be worth updating the readme. I cannot reproduce the 7-times speadup on julia v0.6

julia> VERSION
v"0.6.2"
julia> using Yeppp
julia> x = rand(10^7)
julia> ty = @elapsed Yeppp.log!(similar(x), x)
julia> t  = @elapsed log.(x)
julia> t/ty
0.66288917164934

[KristofferC]

Updated.