update README

README.md

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 # Mortar3D.jl
-Discrete mortar projections in 3D solid mechanics
+
+[![Build Status](https://travis-ci.org/JuliaFEM/Mortar3D.jl.svg?branch=master)](https://travis-ci.org/JuliaFEM/Mortar3D.jl)[![Coverage Status](https://coveralls.io/repos/github/JuliaFEM/Mortar3D.jl/badge.svg?branch=master)](https://coveralls.io/github/JuliaFEM/Mortar3D.jl?branch=master)[![](https://img.shields.io/badge/docs-stable-blue.svg)](https://juliafem.github.io/Mortar3D.jl/stable)[![](https://img.shields.io/badge/docs-latest-blue.svg)](https://juliafem.github.io/Mortar3D.jl/latest)[![Issues](https://img.shields.io/github/issues/JuliaFEM/Mortar3D.jl.svg)](https://github.com/JuliaFEM/Mortar3D.jl/issues)
+
+Mortar3D.jl is a Julia package to calculate discrete projections between
+non-conforming finite element mesheds. The resulting "mortar matrices" can
+be used to tie non-conforming finite element meshes together which are meshed
+separately to construct bigger models.
+
+Using mortar methods in mesh tie problems results variationally consistent
+solution. Mathematically, goal is to solve mixed problem with primary field
+variable and Lagrange multipliers, which have a physical meaning (e.g. contact
+pressure if unknown field is displacement). The problem arising is a typical
+saddle point problem with zeros on diagonal.
+
+## Installing and testing package
+
+Installing package goes same way like other packages in julia, i.e.
+```julia
+julia> Pkg.add("Mortar3D")
+```
+
+Testing package can be done using `Pkg.test`, i.e.
+```julia
+julia> Pkg.test("Mortar3D")
+```
+
+Probably the easiest way to test the functionality of package is to
+use [JuliaBox](https://juliabox.com/).