Nokia RX-51: Add README.nokia_rx51 file to MAINTAINERS

This entry was missing in MAINTAINERS file.

Signed-off-by: Pali Rohár <pali@kernel.org>
diff --git a/board/nokia/rx51/MAINTAINERS b/board/nokia/rx51/MAINTAINERS
index 4cf3dcc..f2a7126 100644
--- a/board/nokia/rx51/MAINTAINERS
+++ b/board/nokia/rx51/MAINTAINERS
@@ -4,3 +4,4 @@
 F:	board/nokia/rx51/
 F:	include/configs/nokia_rx51.h
 F:	configs/nokia_rx51_defconfig
+F:	doc/README.nokia_rx51