doc: arch: Convert README.mips 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>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/arch/index.rst b/doc/arch/index.rst
index a03ee6b..1aeb7a1 100644
--- a/doc/arch/index.rst
+++ b/doc/arch/index.rst
@@ -5,3 +5,5 @@
 
 .. toctree::
    :maxdepth: 2
+
+   mips