README.md: Update contributing instruction (#218)

bekicot · pkuczynski · commit c1a3be11f486 · 2019-04-16T12:00:15.000+02:00
diff --git a/README.md b/README.md
@@ -439,6 +439,13 @@ Settings.section.ssl_enabled # => false
 
 ## Contributing
 
+Install appraisal
+
+```bash
+gem install bundler -v 1.17.3
+gem install appraisal
+```
+
 Bootstrap
 
 ```bash
