doc: hello world: move to rst and expand docuemntation

Change-Id: Id623dd8d2f97836c0f970424d06d9d549065f7c5
Signed-off-by: Anas Nashif <anas.nashif@intel.com>

samples/hello_world/README.rst

@@ -0,0 +1,35 @@
+Hello World
+###########
+
+Overview
+========
+A simple Hello World example that can be used with any supported board and
+prints 'Hello World' to the console. This application can be built into modes:
+
+* single thread
+* multi threading
+
+Building and Running
+====================
+
+This project outputs 'Hello World' to the console.  It can be built and executed
+on QEMU as follows:
+
+.. code-block:: console
+
+    make qemu
+
+
+To build the single thread version, use the supplied configuration file for
+single thread: :file:`prj_single.conf`:
+
+.. code-block:: console
+
+   make CONF_FILE=prj_single.conf qemu
+
+Sample Output
+-------------
+
+.. code-block:: console
+
+    Hello World! x86

samples/hello_world/README.txt

@@ -1,18 +0,0 @@
-Title: Hello World
-
-Description:
-
-A simple Hello World example
-
---------------------------------------------------------------------------------
-
-Building and Running Project:
-
-This project outputs 'Hello World' to the console.  It can be built and executed
-on QEMU as follows:
-
-    make qemu
-
-Sample Output:
-
-Hello World! x86