Add rules section to documentation

docs/create_rules.md

@@ -0,0 +1,50 @@
+# Create rules
+
+In order to lint and score models, `dbt-score` uses a set of rules that is
+applied to each model. A rule can pass or fail when it is applied to a model.
+Based on the severity of the rule models are scored. `dbt-score` has a set of
+rules enabled by default, which can be found [here](reference/rules/generic.md).
+
+On top of the generic rules, it's possible to add your own rules. By default
+`dbt-score` will look for rules in the namespace `dbt_score_rules`.
+
+Two ways exist to create a new rule:
+
+1. By using the `@rule` decorator, which is the preferred/simple way.
+2. By inheriting from the `Rule` class, which is more advanced.
+
+### Using the `@rule` decorator
+
+The `@rule` decorator can be used to easily create a new rule:
+
+```python
+@rule
+def has_description(model: Model) -> RuleViolation | None:
+    """A model should have a description."""
+    if not model.description:
+        return RuleViolation(message="Model lacks a description.")
+```
+
+The severity of a rule can be set using the `severity` argument:
+
+```python
+@rule(severity=Severity.HIGH)
+```
+
+### Using the `Rule` class
+
+For more advanced use cases, a rule can be created by inheriting from the `Rule`
+class:
+
+```python
+class HasDescription(Rule):
+    description = "A model should have a description."
+
+    def evaluate(self, model: Model) -> RuleViolation | None:
+        """Evaluate the rule."""
+        if not model.description:
+            return RuleViolation(message="Model lacks a description.")
+```
+
+Using the rule class can be useful if the rule needs to keep state between
+evaluations.

mkdocs.yml

@@ -6,6 +6,7 @@ plugins:
   - mkdocstrings
 nav:
   - Home: index.md
+  - Create rules: create_rules.md
   - Reference:
       - reference/cli.md
       - reference/config.md