doc: usage: Convert README.plan9 to reST

This converts the existing README.plan9 to reST, and puts it under
the doc/usage/os directory.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index a16deee..8a54d9c 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -60,4 +60,5 @@
 .. toctree::
    :maxdepth: 1
 
+   os/plan9
    os/vxworks
diff --git a/doc/README.plan9 b/doc/usage/os/plan9.rst
similarity index 88%
rename from doc/README.plan9
rename to doc/usage/os/plan9.rst
index 2d3d0e0..f91712c 100644
--- a/doc/README.plan9
+++ b/doc/usage/os/plan9.rst
@@ -1,3 +1,10 @@
+.. SPDX-License-Identifier: GPL-2.0+
+.. Steven Stallion
+.. June 2013
+
+Plan 9
+======
+
 Plan 9 from Bell Labs kernel images require additional setup to pass
 configuration information to the kernel.  An environment variable named
 confaddr must be defined with the same value as CONFADDR (see mem.h).
@@ -10,9 +17,6 @@
 
 If no command line arguments or bootargs are defined, CONFADDR is left
 uninitialized to permit manual configuration.  For example, PC-style
-configuration could be simulated by issuing a fatload in bootcmd:
+configuration could be simulated by issuing a fatload in bootcmd::
 
   # setenv bootcmd fatload mmc 0 $confaddr plan9.ini; ...; bootm
-
-Steven Stallion
-June 2013