Update README.md

Author: junyuan-chen

README.md

@@ -1,6 +1,11 @@
 # DiffinDiffsBase.jl
 
-Base package for [DiffinDiffs.jl](https://github.com/JuliaDiffinDiffs/DiffinDiffs.jl)
+*Base package for [DiffinDiffs.jl](https://github.com/JuliaDiffinDiffs/DiffinDiffs.jl)*
 
 [![CI-stable](https://github.com/JuliaDiffinDiffs/DiffinDiffsBase.jl/workflows/CI-stable/badge.svg)](https://github.com/JuliaDiffinDiffs/DiffinDiffsBase.jl/actions?query=workflow%3ACI-stable)
 [![codecov](https://codecov.io/gh/JuliaDiffinDiffs/DiffinDiffsBase.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/JuliaDiffinDiffs/DiffinDiffsBase.jl)
+
+This package provides types, functions and macros
+that are shared among other component packages of
+[DiffinDiffs.jl](https://github.com/JuliaDiffinDiffs/DiffinDiffs.jl).
+It is not intended to be used as a standalone package.