imx8m: imx8mn-beacon: Migrate README to rst

Since U-Boot builds HTML documentation, migrate the contents
of the README file to an rst file which can generate the
proper outputs.

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/doc/board/beacon/index.rst b/doc/board/beacon/index.rst
index 8eba5e8..bf62b09 100644
--- a/doc/board/beacon/index.rst
+++ b/doc/board/beacon/index.rst
@@ -8,3 +8,4 @@
 
    beacon-imx8mp
    beacon-imx8mm
+   beacon-imx8mn