feat: add readme for errors severities

Author: Artur-Wisniewski

README.md

@@ -227,6 +227,24 @@ custom_lint:
       some_parameter: "some value"
 ```
 
+#### Overriding lint error severities
+
+You can also override the severity of lint rules in the `analysis_options.yaml` file.
+This allows you to change INFO level lints to WARNING or ERROR, or vice versa:
+
+```yaml
+custom_lint:
+  rules:
+    - my_lint_rule # enable the rule with default severity
+  errors:
+    my_lint_rule: error # Override severity to ERROR
+    another_rule: warning # Override severity to WARNING 
+    third_rule: info # Override severity to INFO
+    fourth_rule: none # Suppress the lint entirely
+```
+
+The available severity levels are: `error`, `warning`, `info`, and `none`.
+
 ### Obtaining the list of lints in the CI
 
 Unfortunately, running `dart analyze` does not pick up our newly defined lints.