add .tar.tstamp files to year and top levels

These `.tar.tstamp` files will be used, along with the entry
`.tar.tstamp` files, to form compressed tarballs that
can be reproduced exactly the same way, down to the byte,
under both macOS and Linux.

Stay tuned ... :-)

2011/.filelist

@@ -5,4 +5,5 @@
 2011/Makefile
 2011/README.md
 2011/rules.txt
+2011/.tar.tstamp
 2011/.year