doc: usage: Convert README.vxworks to reST

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

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 5bfe31a..a16deee 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -53,3 +53,11 @@
    cmd/true
    cmd/ums
    cmd/wdt
+
+Booting OS
+----------
+
+.. toctree::
+   :maxdepth: 1
+
+   os/vxworks