Merge pull request #83 from paultcochrane/pr/add-obsolescence-note

Add note about archival nature of repo

README

@@ -1,3 +1,13 @@
+**NOTE**:
+
+This repository is retained only for archival purposes.  The project is
+currently considered dead and only really useful for updating the
+documentation on https://doc.perl6.org.  If you are looking for documentation
+about Perl 6, please search on http://doc.perl6.org and/or check out its
+GitHub project: https://github.com/perl6/doc
+
+======================================================================
+
 We are writing a book about Perl 6.
 
 It will be some kind of example-driven introduction to Perl 6, and at