Add brief project description to the README

Author: ctrueden

README.md

@@ -1,2 +1,8 @@
 [![Build Status](https://github.com/scijava/scijava-links/actions/workflows/build.yml/badge.svg)](https://github.com/scijava/scijava-links/actions/workflows/build.yml)
 
+This package provides a subsystem for SciJava applications
+that enables handling of URI-based links via plugins.
+
+It is kept separate from the SciJava Common core classes
+because it requires Java 11 as a minimum, due to its use
+of java.awt.Desktop features not present in Java 8.