Change references of README.text to README.md

Some manifest files as well as years-spoiler.html and years.html
referred to README.text which commit
df5d583c60 broke.

2012/MANIFEST

@@ -3,7 +3,7 @@
 2012/Makefile
 2012/README.html
 2012/README.markdown
-2012/README.text
+2012/README.md
 2012/blakely/Makefile
 2012/blakely/blakely.c
 2012/blakely/hint.html