update README (actually after update of component);

--- a/Admin/isabelle_fonts/README	Sun Mar 21 23:03:31 2021 +0100
+++ b/Admin/isabelle_fonts/README	Sun Mar 21 23:05:17 2021 +0100
@@ -15,9 +15,12 @@
 of a licence; fonts and documents in this site are free for any use; George
 Douros".
 
+The blackboard-bold glyphs are from font "txmia" (package "pxfonts"):
+these are subject to GPL.
+
 
     Makarius
-    19-Mar-2021
+    21-Mar-2021
 
 
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