Update readme

README.md

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 [![Dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://common.pages.km3net.de/KM3io.jl/dev)
 [![Build Status](https://git.km3net.de/common/KM3io.jl/badges/main/pipeline.svg)](https://git.km3net.de/common/KM3io.jl/pipelines)
 [![Coverage](https://git.km3net.de/common/KM3io.jl/badges/main/coverage.svg)](https://git.km3net.de/common/KM3io.jl/commits/main)
+
+`KM3io.jl` is an pure Julia library which implements I/O functions and utilities
+to deal with dataformats used in KM3NeT.
+
+## Installation
+
+`KM3Acoustics.jl` is not an officially registered Julia package but it's
+available via the KM3NeT Julia registry. To add the KM3NeT Julia registry,
+execute once:
+
+    git clone https://git.km3net.de/common/julia-registry ~/.julia/registries/KM3NeT
+
+Once the registry is added, Julia will make sure to keep it up to date and pick
+it whenever you install a package which is registered there.
+
+To install `KM3io.jl`:
+
+    julia> import Pkg; Pkg.add("KM3io")