docs: custom zephyr application

Add the documentation about how to use Ocre Runtime in an existing
custom Zephyr application.

Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>

Author: casaroli

docs/CustomZephyrApplication.md

@@ -0,0 +1,58 @@
+## Adding Ocre Zephyr module to a custom Zephyr application
+
+Ocre comes with a few samples to get you going fast:
+
+- [`src/samples/mini/zephyr`](samples/mini.md)
+- [`src/samples/demo/zephyr`](samples/demo.md)
+- [`src/samples/supervisor/zephyr`](samples/supervisor.md)
+
+However in most real-world scenarios, you will want to create your own application that controls Ocre.
+
+This will guide you through the process of adding Ocre to an already existing Zephyr application.
+
+Assuming you have a working zephyr build workspace, for example:
+
+```sh
+cd ~/zephyrproject
+ls -1a
+```
+
+Output includes:
+
+```
+.west
+zephyr
+modules
+```
+
+You can add Ocre as a Zephyr module alongside the other directories (or anywhere else):
+
+```sh
+cd ~/zephyrproject
+git clone --recursive https://github.com/project-ocre/ocre-runtime.git
+```
+
+And we can add include the Ocre Zephyr module in the build:
+
+```sh
+export EXTRA_ZEPHYR_MODULES=~/zephyrproject/ocre-runtime
+```
+
+Note that the ocre command line shell interface is not part of ocre-runtime, but part of the supervisor sample.
+
+You probably want your application to control Ocre, being able to `#include <ocre/ocre.h>` and make calls to its API, so you need to tell Zephyr build system to make Ocre Runtime available to your application:
+
+Edit `CMakeLists.txt` on your application and add:
+
+```
+target_link_libraries(app
+    PUBLIC
+    OcreCore
+)
+```
+
+And you can continue building your application normally, which should be able to use Ocre.
+
+See [Get Started with Zephyr](GetStartedZephyr.md) for more information.
+
+Also, check the API documentation for more information on how to use Ocre in your application.