Added dev instructions [skip ci]

Author: ankane

README.md

@@ -619,3 +619,12 @@ Everyone is encouraged to help improve this project. Here are a few ways you can
 - Fix bugs and [submit pull requests](https://github.com/ankane/chartkick/pulls)
 - Write, clarify, or fix documentation
 - Suggest or add new features
+
+To get started with development:
+
+```sh
+git clone https://github.com/ankane/chartkick.git
+cd chartkick
+bundle install
+bundle exec rake test
+```