doc: arch: Convert README.nios2 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/arch/index.rst b/doc/arch/index.rst
index 7f10df0..000f5de 100644
--- a/doc/arch/index.rst
+++ b/doc/arch/index.rst
@@ -9,4 +9,5 @@
    arm64
    mips
    nds32
+   nios2
    x86