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.

2020/.filelist

@@ -0,0 +1,14 @@
+2020/2020.tar.bz2
+2020/MANIFEST
+2020/Makefile
+2020/README.md
+2020/.filelist
+2020/.gitignore
+2020/guidelines.txt
+2020/hint.css
+2020/iocccsize.c
+2020/iocccsize.mk
+2020/iocccsize-test.sh
+2020/rules.txt
+2020/.year
+var.mk