add docs for ports and labels (#175)

Author: baronfel

docs/ContainerCustomization.md

@@ -79,14 +79,47 @@ By default, we use the `/app` directory as the working directory.
 <ContainerWorkingDirectory>/bin</ContainerWorkingDirectory>
 ```
 
+## ContainerPort
+
+This item adds TCP or UDP ports to the list of known ports for the container. This enables container runtimes like Docker to map these ports to the host machine automatically. This is often used as documentation for the container, but can also be used to enable automatic port mapping.
+
+ContainerPort items have two properties:
+* Include
+  * The port number to expose
+* Type
+  * One of `tcp` or `udp` - the default is `tcp`
+
+```xml
+<ItemGroup>
+    <ContainerPort Include="80" Type="tcp" />
+</ItemGroup>
+```
+
+> **Note**
+> This item does nothing for the container by default and should be considered advisory at best.
+
+## ContainerLabel
+
+This item adds a metadata label to the container. Labels have no impact on the container at runtime, but are often used to store version and authoring metadata for use by security scanners and other infrastructure tools.
+
+ContainerLabel items have two properties:
+* Include
+  * The key of the label
+* Value
+  * The value of the label - this may be empty
+
+```xml
+<ItemGroup>
+    <ContainerLabel Include="org.contoso.businessunit" Value="contoso-university" />
+<ItemGroup>
+```
+
 ## Unsupported properties
 
 There are many other properties and items that we want to add support for in subsequent previews:
 
 * Entrypoints
 * Entrypoint Arguments
-* Ports
 * Environment Variables
-* Labels
 
 We expect to add them in future versions, so watch this space!