meta-ohos-demo/README.md: Add small documentation on the dashboard demo

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

meta-ohos-demo/README.md

@@ -16,6 +16,27 @@ The `meta-ohos-demo` layer depends on the following layers:
 Note that the dependencies of the above layers should also be taken into
 consideration.
 
+# Demos
+
+## Dashboard demo
+
+The dashboard demo is composed out of a set of devices that create a home automation demo environment.
+
+### The Gateway device
+
+* Kernel type: linux
+* Hardware (MACHINE): stm32mp1-av96
+* Distribution configuration (DISTRO): openharmony-linux-demo-dashboard
+* Relevant/supported images:
+  * demo-dashboard-gateway-image
+
+Build steps:
+
+1. initialise a Linux flavour build environment as defined in the [meta-ohos documentation](https://git.ostc-eu.org/OSTC/OHOS/meta-ohos#ohos-linux-flavour)
+2. set MACHINE and DISTRO either in your `local.conf` file or pass them as environemnt variables bellow
+3. build image
+  * eg. `bitbake demo-dashboard-gateway-image`
+
 # Contributing
 
 ## Merge requests