Fix #192: Add analyzer.plugins setup to documentation and test configuration

Author: milkyway-12

README.md

@@ -30,6 +30,10 @@ Add an `analysis_options.yaml` file under the `test/` directory, and include the
 
 ```yaml
 include: package:solid_lints/analysis_options_test.yaml
+
+analyzer:
+  plugins:
+    - custom_lint
 ```
 
 Then you can see suggestions in your IDE or you can run checks manually:

lib/analysis_options_test.yaml

@@ -1,5 +1,9 @@
 include: package:solid_lints/analysis_options.yaml
 
+analyzer:
+  plugins:
+    - custom_lint
+
 custom_lint:
   rules:
 # Tests usually organized in one large main() function making this rule not applicable.