Illustrate "filter" attribute with an example

The example was taken from aa4ed402c9
(Add 'filter' attribute and external filter driver definition).

Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -197,6 +197,25 @@ intent is that if someone unsets the filter driver definition,
 or does not have the appropriate filter program, the project
 should still be usable.
 
+For example, in .gitattributes, you would assign the `filter`
+attribute for paths.
+
+------------------------
+*.c	filter=indent
+------------------------
+
+Then you would define a "filter.indent.clean" and "filter.indent.smudge"
+configuration in your .git/config to specify a pair of commands to
+modify the contents of C programs when the source files are checked
+in ("clean" is run) and checked out (no change is made because the
+command is "cat").
+
+------------------------
+[filter "indent"]
+	clean = indent
+	smudge = cat
+------------------------
+
 
 Interaction between checkin/checkout attributes
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^