update readme

Author: bramtayl

README.md

@@ -2,3 +2,5 @@
 
 [![Dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://bramtayl.github.io/MetaGraphsNext.jl/dev)
 [![CodeCov](https://codecov.io/gh/bramtayl/MetaGraphsNext.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/bramtayl/MetaGraphsNext.jl)
+
+This is an experimental, type-stable replacement for [MetaGraphs](https://github.com/JuliaGraphs/MetaGraphs.jl)