add compatibility note about old Julia versions (#36)

README.md

@@ -2,7 +2,8 @@
 
 FileIO.jl integration for image files
 
-[![Build Status](https://travis-ci.org/JuliaIO/ImageIO.jl.svg?branch=master)](https://travis-ci.org/JuliaIO/ImageIO.jl)
+![Julia version](https://img.shields.io/badge/julia-%3E%3D%201.3-blue)
+[![Run tests](https://github.com/JuliaIO/ImageIO.jl/actions/workflows/test.yml/badge.svg)](https://github.com/JuliaIO/ImageIO.jl/actions/workflows/test.yml)
 [![Codecov](https://codecov.io/gh/JuliaIO/ImageIO.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/JuliaIO/ImageIO.jl)
 
 Currently provides:
@@ -30,3 +31,10 @@ load("test.ppm")
 save("test.tiff", rand(RGB, 100, 100))
 load("test.tiff")
 ```
+
+## Compatibility
+
+This package requires Julia at least v1.3. For old Julia versions, a dummy ImageIO version v0.0.1 with no real function will be installed.
+In this case, you still need to install [ImageMagick.jl] to make `FileIO.save`/`FileIO.load` work.
+
+[ImageMagick.jl]: https://github.com/JuliaIO/ImageMagick.jl