Add NEWS item about mpileup zero coverage '*' fixes

Mention the output changes due to b4e452c
(see PR samtools#537); they're surely minor, but it turns out they've been noticed:
https://sourceforge.net/p/samtools/mailman/message/35102141/ (samtools-help,
"Question relating to samtools mpileup output", 2016-05-19).

NEWS

@@ -7,6 +7,12 @@ Noteworthy changes in samtools:
   sites with zero depth; repeating the option as -aa or -a -a additionally
   shows reference sequences without any reads mapped to them (#496).
 
+* The mpileup text output no longer contains empty columns at zero coverage
+  positions.  Previously it would output "...0\t\t..." in some circumstances
+  (zero coverage due to being below a minumum base quality); this has been
+  fixed to output as "...0\t*\t*..." with placeholder '*' characters as in
+  other zero coverage circumstances (see PR #537).
+
 * To stop it from creating too many temporary files, samtools sort
   will now not run unless its per-thread memory limit (-m) is set to
   at least 1 megabyte (#547).