help: explain effect of .hgignore on tracked files

help: explain effect of .hgignore on tracked files

--- a/mercurial/help/hgignore.txt	Sun Jul 01 10:06:16 2012 +0200
+++ b/mercurial/help/hgignore.txt	Mon Jul 02 00:31:22 2012 +0200
@@ -33,6 +33,11 @@
 commands support the ``-I`` and ``-X`` options; see
 :hg:`help <command>` and :hg:`help patterns` for details.
 
+Files that are already tracked are not affected by .hgignore, even
+if they appear in .hgignore. An untracked file X can be explicitly
+added with :hg:`add X`, even if X would be excluded by a pattern
+in .hgignore.
+
 Syntax
 ------