doc: board: Convert README.qemu-riscv 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 a2b1a60..7537179 100644
--- a/doc/board/emulation/index.rst
+++ b/doc/board/emulation/index.rst
@@ -7,4 +7,5 @@
    :maxdepth: 2
 
    qemu-arm
+   qemu-riscv
    qemu-x86