No mention of the form of "f" in the README

[huanglangwen]

I ran into an error when using the inplace jacobian. I later discovered that's due to the function have to be in the form of (fx,x)->(fx=f(x)). I think this should be mentioned on the document.

julia> using DiffEqDiffTools
julia> jac_cache=DiffEqDiffTools.JacobianCache(zeros(2),Val{:forward})
julia> J=zeros((2,2))
julia> DiffEqDiffTools.finite_difference_jacobian!(J,x->(x.=x.+1;x),[0.,0.],jac_cache)

ERROR: MethodError: no method matching (::getfield(Main, Symbol("##3#4")))(::Array{Flo
at64,1}, ::Array{Float64,1})
Closest candidates are:
  #3(::Any) at REPL[4]:1
Stacktrace:
 [1] finite_difference_jacobian!(::Array{Float64,2}, ::getfield(Main, Symbol("##3#4"))
, ::Array{Float64,1}, ::DiffEqDiffTools.JacobianCache{Array{Float64,1},Array{Float64,1
},Array{Float64,1},Val{:forward},Float64,Val{true}}, ::Nothing) at /root/.julia/packag
es/DiffEqDiffTools/bzjuh/src/jacobians.jl:121
 [2] finite_difference_jacobian!(::Array{Float64,2}, ::Function, ::Array{Float64,1}, :
:DiffEqDiffTools.JacobianCache{Array{Float64,1},Array{Float64,1},Array{Float64,1},Val{
:forward},Float64,Val{true}}) at /root/.julia/packages/DiffEqDiffTools/bzjuh/src/jacob
ians.jl:109
 [3] top-level scope at none:0

[ChrisRackauckas]

Yeah, that's quite an omission from the README. Thanks for pointing it out!

[ChrisRackauckas]

added