Documentation of new keyword instantiate (COVESA#461)

As discussed in COVESA#448 and implemented in
COVESA/vss-tools#168
it gives more flexibility in defining instances.

Signed-off-by: Daniel Wilms <Daniel.DW.Wilms@bmw.de>

docs-gen/content/rule_set/instances.md

@@ -18,6 +18,8 @@ interpreted by the tools.
 
 ## Definition
 
+### How can I instances for my `branch`?
+
 1. An instance can be defined in any branch.
 2. The instantiation is done for every node in the following path.
 3. Instances are defined with the key-word `instances`, followed by its
@@ -35,6 +37,19 @@ interpreted by the tools.
    four position instances will be created for each of the 'Left' and 'Right' instances,
    resulting into a total number of 8 instances.
 
+### How can I exclude child-nodes from instantiation?
+
+Often it makes sense to instantiate all child-nodes of a branch.
+But there are cases, when nodes are linked more the general concept of
+a branch, but not to the single instance. This could be the `DoorCount`,
+which would rather be `Door.Count`, `WheelDiamater`, which is rather linked
+to an axle rather than the wheel itself or `Brake.FluidLevel` which is not
+measured for a single break, but rather a system indication. 
+
+To exclude a child-node from the instantiation of the *direct* parent node, set the
+keyword `instantiate` to `false` (`true` by default). Please check the following
+example for details. 
+
 ## Example
 
 The example from above in the specification:
@@ -49,7 +64,16 @@ Door:
   description: All doors, including windows and switches
 #include SingleDoor.vspec Door
 
+Door.Count:
+  datatype: uint8
+  type: attribute
+  default: 4
+  instantiate: false
+  description: Number of doors in vehicle.
+```
+
 
+```yml
 # SingleDoor.vspec
 
 #
@@ -65,6 +89,7 @@ Results in the following dot-notated output:
 
 ```
 Vehicle.Cabin.Door
+Vehicle.Cabin.Door.Count
 Vehicle.Cabin.Door.Row1
 Vehicle.Cabin.Door.Row1.Left
 Vehicle.Cabin.Door.Row1.Left.IsOpen