Update README.md

README.md

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 # BcubeCGNS.jl
-Implementation of Bcube IO interface for CGNS format
+Implementation of [`Bcube`](https://github.com/bcube-project/Bcube.jl) IO interface for CGNS format. Checkout the relative `Bcube` [documentation](https://bcube-project.github.io/Bcube.jl/stable/api/io/io_interface/) for more infos.
+
+Note that all CGNS specifications are not implemented (for instance, NGON elements).
+
+## Basic usage
+```julia
+using Bcube
+using BcubeCGNS
+
+mesh = rectangle_mesh(10, 20)
+U = TrialFESpace(FunctionSpace(:Lagrange, 1), mesh)
+u = FEFunction(U)
+projection_l2!(u, PhysicalFunction(x -> sum(x)), CellDomain(mesh))
+
+write_file("output.cgns", mesh, Dict("u" => u, "grad_u" => ∇(u)))
+
+result = read_file("output.cgns"; varnames = "*")
+mesh = result.mesh
+data = result.data # Dict of variable name (String) to MeshData
+```