Add documentation to custom section of Porter manifest

Signed-off-by: joshuabezaleel <joshua.bezaleel@gmail.com>

docs/content/bundle/manifest/_index.md

@@ -500,6 +500,7 @@ data.
 
 ```yaml
 custom:
+  custom-config: "custom-value"
   some-custom-config:
     item: "value"
   more-custom-config:
@@ -511,6 +512,12 @@ You can access custom data at runtime using the `bundle.custom.KEY.SUBKEY` templ
 For example, `{{ bundle.custom.more-custom-config.enabled}}` allows you to
 access nested values from the custom section.
 
+Multiple custom values that were defined at the manifest can also be injected with new values during build time using the \--custom values tied to the `porter build` command. Currently only support string value. You can use dot notation to specify a nested field:
+
+```
+porter build --custom custom-config=new-custom-value --custom some-custom-config.item=edited-value
+```
+
 See the [Custom Extensions](https://github.com/cnabio/cnab-spec/blob/master/101-bundle-json.md#custom-extensions)
 section of the CNAB Specification for more details.