Minor clarification for quick start guide.

Added a note for toolchain selection while using `build_default.py`
in `docs/quick_start.md`

Change-Id: I3d7405ff3257ec9d0358eb44d5d4c1feeb39a047
diff --git a/docs/quick_start.md b/docs/quick_start.md
index 6aea7b1..3efbd52 100644
--- a/docs/quick_start.md
+++ b/docs/quick_start.md
@@ -28,7 +28,15 @@
     neural network operators should be [supported by Vela](https://review.mlplatform.org/plugins/gitiles/ml/ethos-u/ethos-u-vela/+/HEAD/SUPPORTED_OPS.md).
 
     ```commandline
-    python3 build_default.py
+    ./build_default.py
+    ```
+
+    > **Note** The above command assumes you are using the GNU Arm Embedded Toolchain.
+    > If you are using the Arm Compiler instead, you can override the default selection
+    > by executing:
+
+    ```commandline
+    ./build_default.py --toolchain arm
     ```
 
 5. Launch the project as explained [here](sections/deployment.md#Deployment). For the purpose of this quick start guide,