doc: board: Convert README.qemu-arm to reST

Convert plain text documentation to reStructuredText format and add
it to Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/board/emulation/index.rst b/doc/board/emulation/index.rst
index 6d533f3..a2b1a60 100644
--- a/doc/board/emulation/index.rst
+++ b/doc/board/emulation/index.rst
@@ -6,4 +6,5 @@
 .. toctree::
    :maxdepth: 2
 
+   qemu-arm
    qemu-x86