help: add documentation on include: and subinclude:

Adds some help text about the new include: and subinclude: match syntax.

mercurial/help/hgignore.txt

@@ -68,6 +68,10 the form ``*.c`` will match a file endin
 and a regexp pattern of the form ``\.c$`` will do the same. To root a
 regexp pattern, start it with ``^``.
 
+Subdirectories can have their own .hgignore settings by adding
+``subinclude:path/to/subdir/.hgignore`` to the root ``.hgignore``. See
+:hg:`help patterns` for details on ``subinclude:`` and ``include:``.
+
 .. note::
 
   Patterns specified in other than ``.hgignore`` are always rooted.

mercurial/help/patterns.txt

@@ -30,6 +30,12 The latter expects null delimited patter
 feeds. Each string read from the file is itself treated as a file
 pattern.
 
+To read a set of patterns from a file, use ``include:`` or ``subinclude:``.
+``include:`` will use all the patterns from the given file and treat them as if
+they had been passed in manually.  ``subinclude:`` will only apply the patterns
+against files that are under the subinclude file's directory. See :hg:`help
+hgignore` for details on the format of these files.
+
 All patterns, except for ``glob:`` specified in command line (not for
 ``-I`` or ``-X`` options), can match also against directories: files
 under matched directories are treated as matched.
@@ -60,3 +66,9 File examples::
   listfile0:list.txt read list from list.txt with null byte delimiters
 
 See also :hg:`help filesets`.
+
+Include examples::
+
+  include:path/to/mypatternfile    reads patterns to be applied to all paths
+  subinclude:path/to/subignorefile reads patterns specifically for paths in the
+                                   subdirectory