Add free access atrribute

docs/attribute_guidelines.md

@@ -58,4 +58,12 @@ Those attributes will be validated with unit tests when used.
         <td><code>List[str]</code></td>
         <td><code>generic_topic_parsing</code></td>
     </tr>
+    <tr>
+        <td>free_access</td>
+        <td>A boolean which is set to be False, if the article is restricted to users with a subscription. This usually indicates
+        that the article cannot be crawled completely.
+        <i>This attribute is implemented by default</i></td>
+        <td><code>bool</code></td>
+        <td><code></code></td>
+    </tr>
 </table>

docs/how_to_add_a_publisher.md

@@ -469,6 +469,24 @@ Instead, we recommend referring to [this](https://devhints.io/xpath) documentati
 Make sure to examine other parsers and consult the [attribute guidelines](attribute_guidelines.md) for specifics on attribute implementation. 
 We strongly encourage utilizing these utility functions, especially when parsing the `ArticleBody`.
 
+### Checking the free_access attribute
+
+In case your new publisher does not have a subscription model, you can go ahead and skip this step. If it does,
+please verify that there is a tag `isAccessibleForFree` within the `<script type="application/ld+json">` blocks in the
+source code of premium articles that is set to either `false` or `False`. It doesn't matter if the tag is missing in the
+freely accessible articles. If this is the case, you can continue with the next step. If not, please overwrite the
+existing function by adding the following snippet to your parser:
+
+```python
+@attribute
+def free_access(self) -> bool:
+    # Your personalized logic goes here
+    pass
+```
+
+Usually you can identify a premium article by an indicator within the URL or by using XPath or CSSSelector and selecting
+the element asking to to purchase a subscription to view the article.
+
 ### Finishing the Parser
 
 Bringing all the above together, the Los Angeles Times now looks like this.