Update README on how to get public cloud image names for VNF packages

Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>

README.md

@@ -166,5 +166,35 @@ Options:
   -h, --help                  Show this message and exit.
 ```
 
+## Using images in public clouds
+
+When a NF package wants to use an image from the public cloud, we need to specify the specific image in that public cloud. The name will be different for each public cloud. That's why we need to specify, for each VIM type, a different image. Find below an excerpt from `hackfest_basic_vnf`:
+
+```yaml
+  sw-image-desc:
+  - id: ubuntu20.04
+    image: ubuntu20.04
+    name: ubuntu20.04
+  - id: ubuntu20.04-aws
+    name: ubuntu20.04-aws
+    image: ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-20231025
+    vim-type: aws
+  - id: ubuntu20.04-azure
+    name: ubuntu20.04-azure
+    image: Canonical:0001-com-ubuntu-server-focal:20_04-lts:latest
+    vim-type: azure
+  - id: ubuntu20.04-gcp
+    name: ubuntu20.04-gcp
+    image: ubuntu-os-cloud:image-family:ubuntu-2004-lts
+    vim-type: gcp
+```
+
+In order to know what are the name of the images on each public cloud, you could check the links below:
 
+- Ubuntu images in Azure: <https://canonical-azure.readthedocs-hosted.com/en/latest/azure-how-to/instances/find-ubuntu-images/>
+- Ubuntu images in GCP: <https://canonical-gcp.readthedocs-hosted.com/en/latest/google-how-to/gce/find-ubuntu-images/>
+- Ubuntu images in AWS: <https://aws.amazon.com/marketplace/pp/prodview-tuap4sdmqpo52>
+  - From AWS Console > EC2 > Images > AMI > Search
+    - Public images
+    - Filter: "Platform=Ubuntu", "Virtualization=hvm", "Architecture=x86_64", "AMI name=ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server"