docs: add a step to declare the ORM mapped class for checks

Signed-off-by: behnazh-w <[email protected]>

Author: behnazh-w

src/macaron/slsa_analyzer/checks/README.md

@@ -11,7 +11,7 @@ These are the steps for creating a Check in Macaron:
 1. Create a module with the name `<name>_check.py`. Note that Macaron **only** loads check modules that have this name format.
 2. Create a class that inherits `BaseCheck` and initiates the attributes of a `BaseCheck` instance.
 3. Register the newly created Check class to the Registry ([registry.py](../registry.py)). This will make the Check available to Macaron. For example:
-```
+```python
 from macaron.slsa_analyzer.registry import registry
 
 # Check class is defined here
@@ -20,5 +20,23 @@ from macaron.slsa_analyzer.registry import registry
 
 registry.register(ExampleCheck())
 ```
+4. Add an ORM mapped class for the check facts so that the policy engine can reason about the properties. To provide the mapped class, all you need to do is to add a class that inherits from `CheckFacts` class and add the following attributes (rename the `MyCheckFacts` check name and `__tablename__` as appropriate).
+
+```python
+class MyCheckFacts(CheckFacts):
+    """The ORM mapping for justifications in my check."""
+
+    __tablename__ = "_my_check"
+
+    #: The primary key.
+    id: Mapped[int] = mapped_column(ForeignKey("_check_facts.id"), primary_key=True)  # noqa: A003
+
+    #: The name of the column (property) that becomes available to policy engine.
+    my_column_name: Mapped[str] = mapped_column(String, nullable=False)
+
+    __mapper_args__ = {
+        "polymorphic_identity": "_my_check",
+    }
+```
 
 For more examples, please see the existing Checks in [checks/](./).