check-mailmap doc: note config options

Add a passing mention of the mailmap.file and mailmap.blob
configuration options. Before this addition a reader of the
"check-mailmap" manpage would have no idea that a custom map could be
specified, unless they'd happen to e.g. come across it in the "config"
manpage first.

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

Documentation/git-check-mailmap.txt

@@ -36,6 +36,13 @@ name is provided or known to the 'mailmap', ``Name $$<user@host>$$'' is
 printed; otherwise only ``$$<user@host>$$'' is printed.
 
 
+CONFIGURATION
+-------------
+
+See `mailmap.file` and `mailmap.blob` in linkgit:git-config[1] for how
+to specify a custom `.mailmap` target file or object.
+
+
 MAPPING AUTHORS
 ---------------