Add maintenance warning to README

Author: juliohm

README.md

@@ -2,6 +2,13 @@
 
 [![Build Status](https://github.com/JuliaGeo/Geodesy.jl/workflows/CI/badge.svg)](https://github.com/JuliaGeo/Geodesy.jl/actions)
 
+> ### ⚠️ This package is not actively maintained
+> 
+> Alternatives with improved design are available:
+>
+> - [CoordRefSystems.jl](https://github.com/JuliaEarth/CoordRefSystems.jl) - Native Julia implementation with built-in support for units, thoroughly tested against the PROJ C library.
+> - [MapMaths.jl](https://github.com/subnero1/MapMaths.jl) - Another Julia implementation with support for the WGS84 datum and local coordinates.
+
 **Geodesy** is a Julia package for working with points in various world and
 local coordinate systems. The primary feature of *Geodesy* is to define and
 perform coordinate transformations in a convenient and safe framework,
@@ -11,11 +18,6 @@ Transformations are accurate and efficient and implemented in native Julia code
 [C++ library](http://geographiclib.sourceforge.net/)), and some common geodetic
 datums are provided for convenience.
 
-## Alternatives
-
-- [CoordRefSystems.jl](https://github.com/JuliaEarth/CoordRefSystems.jl) - Native Julia implementation with built-in support for units, widely tested against the PROJ C library.
-- [MapMaths.jl](https://github.com/subnero1/MapMaths.jl) - Another Julia implementation with support for the WGS84 datum and local coordinates.
-
 ## Quick start
 
 Lets define a 3D point by its latitude, longitude and altitude (LLA):