Distribution: add example for hiding fields from the form.

I am making a distribution for a client.

* I don't want to show the description field because it should remain empty (at least that is how it is on their current Plone 4 site).
* I don't want to show the default_language field, because I am setting this (and four available languages) in the Generic Setup profile.
* Same for the portal_timezone: it is a site for Belgium, so I am setting Europe/Brussels in the Generic Setup profile, so this timezone is always set, even when a colleague in Portugal creates a site on his laptop.

Unfortunately currently these fields have to be in the schema, as documented, otherwise you get an error.
So instead  I found out how to hide these fields.

docs/developer-guide/create-a-distribution.md

@@ -245,6 +245,30 @@ You may have noticed the entries for both `default_language` and `portal_timezon
 `plone.distribution` adds both definitions at runtime, providing a list of languages and timezones available on the installation.
 ````
 
+If you want to hide fields in the form, you can use the `uischema`.
+This is especially useful for fields that must be in the schema, but for which you always want the default value.
+The next lines hide three fields:
+
+```json
+{
+  "schema": {
+    ...
+  },
+  "uischema": {
+      "description": {
+          "ui:widget": "hidden"
+      },
+      "default_language": {
+          "ui:widget": "hidden"
+      },
+      "portal_timezone": {
+          "ui:widget": "hidden"
+      }
+  }
+}
+```
+
+
 ## Add a dependency on an add-on
 
 If you want to add a Plone backend add-on to your Plone distribution, then you must perform the following steps.