Update README.md

README.md

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 [![Build Status](https://travis-ci.com/mzilhao/Jecco.jl.svg?branch=master)](https://travis-ci.com/mzilhao/Jecco.jl)
 
 The **Julia Einstein Characteristic Code** (**Jecco**; [['ʒɛku](https://www.infopedia.pt/dicionarios/lingua-portuguesa/Jeco)]) provides tools to solve Einstein's equations in the characteristic formulation in [Julia](http://julialang.org/).
+
+
+## Installing
+
+This package is not in the registry. You can install it by specifying the URL to the repository:
+
+```julia
+pkg> add https://github.com/mzilhao/Jecco.jl/
+
+julia> using Jecco, Jecco.AdS5_3_1
+```