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>

doc/usage/index.rst

@@ -60,4 +60,5 @@ Booting OS
 .. toctree::
    :maxdepth: 1
 
+   os/plan9
    os/vxworks

doc/README.plan9 → doc/usage/os/plan9.rst

+.. 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 @@ bootargs environment variable will be copied.
 
 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