docs: add example how to share images with other accounts

To make it clear how sharing works and how to restrict sharing by AWS
partition.

Author: toabctl

docs/config-samples/config-minimal-share.yaml

@@ -0,0 +1,12 @@
+awspub:
+  source:
+    path: "image.vmdk"
+    architecture: "x86_64"
+  s3:
+    bucket_name: "awspub-toabctl"
+  images:
+    "my-custom-image":
+      boot_mode: "uefi-preferred"
+      share:
+        - "123456789123"
+        - "aws-cn:456789012345"

docs/how_to/publish.rst

@@ -187,6 +187,18 @@ The image needs to be created and then published:
   awspub create config.yaml
   awspub publish config.yaml
 
+Sharing images
+~~~~~~~~~~~~~~
+
+Images can be shared with other AWS accounts. For that, the account IDs of the other accounts are needed.
+
+.. literalinclude:: ../config-samples/config-minimal-share.yaml
+   :language: yaml
+
+In the above example, the image `my-custom-image` will be shared with the account `1234567890123`
+when `awspub` runs in the commercial partition (``aws``, the default). It'll be shared
+with the account `456789012345` when `awspub` runs in the the china partition (``aws-cn``).
+
 AWS Marketplace
 ~~~~~~~~~~~~~~~