add YYYY/.filelist files

A _YYYY/.filelist_ is a simple list of files for a given IOCCC year
that need to be present when building a _YYYY_ level compressed
tarball.  The list is relative to _docroot_ (top level repo directory).

We do not need a complex JSON file for such a simple list of files.
We only care about checking for the existence of files when forming
_YYYY_ level compressed tarball.

2000/.filelist

@@ -0,0 +1,10 @@
+2000/2000.tar.bz2
+2000/Makefile
+2000/README.md
+2000/.filelist
+2000/.gitignore
+2000/guidelines.txt
+2000/mkentry.c
+2000/rules.txt
+2000/.year
+var.mk