doc: Add a description for bootmeth_android

Add initial documentation for the Android bootmeth.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index a6e47e8..c283234 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -945,6 +945,7 @@
 T:	git https://source.denx.de/u-boot/custodians/u-boot-dfu.git
 F:	boot/bootmeth_android.c
 F:	boot/bootmeth_android.h
+F:	doc/develop/bootstd/android.rst
 
 BTRFS
 M:	Marek Behún <kabel@kernel.org>