update doc/readme with common pitfalls & errors

[dmetivie]

• Bad initilization
• Convergence failure Handling dropouts #11 and Improve robustness #12
• Undefined weighted method like Fitting a beta mixture fails with no method found #9

  [2] suffstats(::Type{Beta{Float64}}, ::Vector{Float64}, ::Vector{Float64})
    @ Distributions C:\Users\metivier\.julia\packages\Distributions\SUTV1\src\genericfit.jl:5
  [3] fit_mle(dt::Type{Beta{Float64}}, x::Vector{Float64}, w::Vector{Float64})