docs/system/devices/igb: Add igb documentation

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 4a6c22b..564af9d 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2252,6 +2252,7 @@
 igb
 M: Akihiko Odaki <akihiko.odaki@daynix.com>
 S: Maintained
+F: docs/system/devices/igb.rst
 F: hw/net/igb*
 F: tests/avocado/igb.py
 F: tests/qtest/igb-test.c