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.

2015/.filelist

@@ -0,0 +1,13 @@
+2015/2015.tar.bz2
+2015/MANIFEST
+2015/Makefile
+2015/README.md
+2015/.filelist
+2015/.gitignore
+2015/guidelines.txt
+2015/hint.css
+2015/iocccsize.c
+2015/iocccsize.mk
+2015/rules.txt
+2015/.year
+var.mk