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/doc/develop/bootstd/index.rst b/doc/develop/bootstd/index.rst
index 9d35b56..4c4e26c 100644
--- a/doc/develop/bootstd/index.rst
+++ b/doc/develop/bootstd/index.rst
@@ -10,6 +10,7 @@
    extlinux
    pxelinux
    qfw
+   android
    cros
    script
    sandbox