add make genpath rules, .path files, updated .top and .year

Add .path files to each winner directory: a file containing
the path to the winner directory from the top directory.

Changed YYYY/.year files to contain the paths, from the top
directory, if the winning directories for the given year.

Changed `/.top` to contain the IOCCC years.

Added `make genpath` rule to the top Makefile and the
year Makefiles to generate the above.

Added FAQ 6.8 to `faq.md` to explain the above.

Updated manifest to reflect recent changes to the list of files.

2015/.year

@@ -1 +1,14 @@
-2015
+2015/burton
+2015/dogon
+2015/duble
+2015/endoh1
+2015/endoh2
+2015/endoh3
+2015/endoh4
+2015/hou
+2015/howe
+2015/mills1
+2015/mills2
+2015/muth
+2015/schweikhardt
+2015/yang