Add segmentField auto-fill for webroot create request

```
@Test
public void testCreateNodeViaPath() {
NodeCreateRequest nodeCreateRequest = new NodeCreateRequest();
nodeCreateRequest.setLanguage("en");
nodeCreateRequest.setSchemaName("content");
nodeCreateRequest.getFields().put("teaser", FieldUtil.createStringField("some teaser"));
//nodeCreateRequest.getFields().put("slug", FieldUtil.createStringField("new-page.html"));
nodeCreateRequest.getFields().put("content", FieldUtil.createStringField("Blessed mealtime again!"));

NodeResponse response = call(() -> client().webrootCreate(PROJECT_NAME, "/new-page.html", nodeCreateRequest));
assertEquals("0.1", response.getVersion());
}
```

* Add test which tests node creation with missing segmentField field
* Add documentation on webroot update / create handling
* Implement segmentField usage for node create requests via webroot