Document package migration to OrdinaryDiffEq.jl

Added update section indicating the package migration to OrdinaryDiffEq.jl.

Author: ChrisRackauckas

README.md

@@ -4,6 +4,13 @@
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+## Update
+
+This package was moved to be a sublibrary in [OrdinaryDiffEq.jl](https://github.com/SciML/OrdinaryDiffEq.jl). The migration is a maintanance change but should have no functional
+change to the library.
+
+## Overview 
+
 StochasticDiffEq.jl is a component package in the DifferentialEquations ecosystem. It holds the
 stochastic differential equations solvers and utilities. While completely independent
 and usable on its own, users interested in using this