add readme

README.md

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+# MLJSerialization.jl 
+
+| Linux | Coverage |
+| :-----------: | :------: |
+| [![Build status](https://github.com/JuliaAI/MLJSerialization.jl/workflows/CI/badge.svg)](https://github.com/JuliaAI/MLJSerialization.jl/actions)| [![codecov.io](http://codecov.io/github/JuliaAI/MLJSerialization.jl/coverage.svg?branch=master)](http://codecov.io/github/JuliaAI/MLJSerialization.jl?branch=master) |
+
+A package providing integration of [Serialization](https://www.openml.org) with the
+[MLJ](https://alan-turing-institute.github.io/MLJ.jl/dev/) machine
+learning framework.
+
+
+## Installation
+
+```julia
+using Pkg
+Pkg.add("MLJ")
+Pkg.add("MLJSerialization")
+```
+
+## Sample usage
+
+Fit and save a decision tree model:
+
+```julia
+Pkg.add("DecisionTree")
+
+using MLJ
+using MLJSerialization
+
+X, y = @load_iris
+
+Tree = @load DecisionTreeClassifier pkg=DecisionTree
+tree = Tree()
+mach = fit!(machine(tree, X, y))
+MLJSerialization.save("my_machine.jlso", mach)
+```
+
+Retrieve the saved machine:
+
+```julia
+mach2 = machine("my_machine.jlso")
+
+Xnew = selectrows(X, 1:3)
+predict_mode(mach2, Xnew)
+julia> predict_mode(mach2, Xnew)
+3-element CategoricalArrays.CategoricalArray{String,1,UInt32}:
+ "setosa"
+ "setosa"
+ "setosa"
+```
+
+## Documentation
+
+Documentation is provided in the [Saving
+machines](https://alan-turing-institute.github.io/MLJ.jl/dev/machines/#Saving-machines-1)
+section of the
+[MLJManual](https://alan-turing-institute.github.io/MLJ.jl/dev/)
+
+