docs/system: Document the arm virt board

Document the arm 'virt' board, which has been undocumented
for far too long given that it is the main recommended board
type for arm guests.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200713175746.5936-5-peter.maydell@linaro.org
diff --git a/MAINTAINERS b/MAINTAINERS
index 935ccb3..5e86168 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -880,6 +880,7 @@
 S: Maintained
 F: hw/arm/virt*
 F: include/hw/arm/virt.h
+F: docs/system/arm/virt.rst
 
 Xilinx Zynq
 M: Edgar E. Iglesias <edgar.iglesias@gmail.com>