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.

years.html

@@ -1021,7 +1021,7 @@ a summary description of all of the winners.  </P>
 <LI><A HREF="2013/2013.tar.bz2">2013.tar.bz2</A>  - all of the data for 2013 (2.3 MiB)
 <LI><A HREF="2013/Makefile">Makefile</A>
 <LI><A HREF="2013/README.html">README.html</A>
-<LI><A HREF="2013/README.text">README.text</A>
+<LI><A HREF="2013/README.md">README.md</A>
 <LI><A HREF="2013/hint.css">hint.css</A>
 <LI><A HREF="2013/whowon.html">whowon.html</A>
 </UL>
@@ -1204,7 +1204,7 @@ a summary description of all of the winners.  </P>
 <LI><A HREF="2012/MANIFEST">MANIFEST</A>
 <LI><A HREF="2012/Makefile">Makefile</A>
 <LI><A HREF="2012/README.html">README.html</A>
-<LI><A HREF="2012/README.text">README.text</A>
+<LI><A HREF="2012/README.md">README.md</A>
 <LI><A HREF="2012/guidelines.txt">guidelines.txt</A>
 <LI><A HREF="2012/hint.css">hint.css</A>
 <LI><A HREF="2012/rules.txt">rules.txt</A>
@@ -1563,7 +1563,7 @@ a summary description of all of the winners.  </P>
 <UL TYPE=square>
 <LI><A HREF="2006/2006.tar.bz2">2006.tar.bz2</A>  - all of the data for 2006 (598 KiB)
 <LI><A HREF="2006/Makefile">Makefile</A>
-<LI><A HREF="2006/README.text">README.text</A>
+<LI><A HREF="2006/README.md">README.md</A>
 <LI><A HREF="2006/guidelines.txt">guidelines.txt</A>
 <LI><A HREF="2006/rules.txt">rules.txt</A>
 </UL>
@@ -1748,7 +1748,7 @@ a summary description of all of the winners.  </P>
 <UL TYPE=square>
 <LI><A HREF="2005/2005.tar.bz2">2005.tar.bz2</A>  - all of the data for 2005 (122 KiB)
 <LI><A HREF="2005/Makefile">Makefile</A>
-<LI><A HREF="2005/README.text">README.text</A>
+<LI><A HREF="2005/README.md">README.md</A>
 <LI><A HREF="2005/guidelines.txt">guidelines.txt</A>
 <LI><A HREF="2005/rules.txt">rules.txt</A>
 </UL>