doc: add workspace initialization into existing NCS workspace

Provide command line instructions on how to add add-on into
existing nRF Connect SDK workspace.

Signed-off-by: Eduardo Montoya <eduardo.montoya@nordicsemi.no>

Author: edmont

docs/setup.rst

@@ -40,13 +40,80 @@ Follow the `Installing the nRF Connect SDK`_ instructions, with the following ex
 
    .. group-tab:: command line
 
-      In the **Get the nRF Connect SDK code** part of the install instructions:
+      Two alternative approaches for initializing the workspace:
+
+      a. **Initialize workspace from scratch**
+
+         In the **Get the nRF Connect SDK code** part of the install instructions:
+
+         In Step 4, use the following command to initialize west with |addon| v\ |addon_version|, which also initializes |NCS| v\ |ncs_version|:
+
+         1. Initialize ncs for the add-on
+
+            .. code-block:: console
+
+               west init -m "http://github.com/nrfconnect/ncs-zigbee" --mr v1.2.1
+
+         #. Update nRF Connect SDK modules
+
+            .. code-block:: console
+
+               west update
+
+      #. **Add add-on into existing nRF Connect SDK workspace**
+
+         Assume you have an existing nRF Connect SDK workspace in the ``ncs`` folder. Run the following commands:
+
+         1. Navigate to the workspace folder
+
+            .. code-block:: console
+
+               cd ncs
+
+         #. Clone add-on repository
+
+            .. code-block:: console
+
+               git clone https://github.com/nrfconnect/ncs-zigbee
+
+         #. Set manifest path to the add-on directory
+
+            .. code-block:: console
+
+               west config manifest.path ncs-zigbee
+
+         #. Update nRF Connect SDK modules
+
+            .. code-block:: console
+
+               west update
+         
+         In case you need to go back to work on the nRF Connect SDK:
+
+         1. Configure the manifest path back to the nRF Connect SDK directory
+
+            .. code-block:: console
+
+               west config manifest.path nrf
+
+         #. Update nRF Connect SDK modules
+
+            .. code-block:: console
+
+               west update
+         
+         You can always check the current manifest path with the following command:
+
+            .. code-block:: console
+
+               west config manifest.path
 
-      In Step 4, use the following command to initialize west with |addon| v\ |addon_version|, which also initializes |NCS| v\ |ncs_version|:
+         The output will be something like:
 
-      .. code-block:: console
+            .. code-block:: console
 
-         west init -m "http://github.com/nrfconnect/ncs-zigbee" --mr v1.2.1
+               nrf
 
+         This means that the current workspace is using the nRF Connect SDK.
 
 To create an application, use :ref:`zigbee_template_sample` sample as a starting point.