libraries.json: update description

Author: jll63

meta/libraries.json

@@ -7,7 +7,7 @@
     "maintainers": [
         "Jean-Louis Leroy"
     ],
-    "description": "Open-methods for C++17 and above.",
+    "description": "Open-methods are free-standing functions that work like virtual functions: they select the b​est overrider from a set, depending on the dynamic type of their arguments. This makes it possible to add polymorphic operations to existing classes, without modifying them. They make patterns like Visitor unnecessary.",
     "category": [
         "Emulation", "Programming"
     ],