Add some comments to README

Author: c42f

README.md

@@ -1,3 +1,17 @@
 # UnicodeNext
 
 [![Build Status](https://github.com/c42f/UnicodeNext.jl/actions/workflows/CI.yml/badge.svg?branch=main)](https://github.com/c42f/UnicodeNext.jl/actions/workflows/CI.yml?query=branch%3Amain)
+
+A unicode library which can be upgraded to the latest unicode rules, even on
+old Julia versions.
+
+The interface here is an amalgamation of Julia's `Unicode` stdlib and the
+functions from Base's strings/unicode.jl. The implementation is a Julia port of
+the C library [utf8proc](https://github.com/JuliaStrings/utf8proc) which has
+traditionally powered Base's unicode functionality.
+
+Ideally I (Claire) hope this library might become the implementation of the
+Unicode stdlib and the Base functions from strings/unicode.jl, such as
+`islowercase()`, etc. But we'd need to figure out the implications of trying to
+make this upgradable without breaking how it's integrated into the Julia
+sysimage.